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ECLIPSES    OF    THE    SUN 


THE  END  OF  THE  TOTAL  ECLIPSE,  SEPTEMBER  10,  1923 
From  the  painting  by  Howard  Russell  Butler,  N.  A.    Shows  the  edge  of  the  reappearing  sun. 


ECLIPSES  OF  THE 

SUN 


BY 


S.  A.  MITCHELL 


PROFESSOR  F"TTflTo*fr6"i?TY  AT 
THE  UNIVERSITY  OF  VIRGINIA 
AND  DIRECTOR  OF  THE  LEANDER 
MCCORMICK  OBSERVATORY 


FOURTH    EDITION 

REVISED    AND    ENLARGED 


NEW    YORK  !!g  M-CM-XXXV 

COLUMBIA     UNIVERSITY     PRESS 


This  edition  comprises  all  the  material  contained  in  the  third  edition, 
as  well  as  a  new  chapter  and  certain  additions  to  the  text. 
Chapters  in  which  no  out-of-date  scientific  statements  occur 
have  been  reprinted  without  change  from  the  third  edition. 


COPYRIGHT  1923,  1924,  1932,  1935 
COLUMBIA  UNIVERSITY  PRESS 


First  edition,  1923 

Second  edition,  revised  and  enlarged,  1924 
Third  edition,  revised  and  enlarged,  1932 
Fourth  edition,  revised  and  enlarged,  1935 


PRINTED  IN  THE  UNITED  STATES  OF  AMERICA 
BY  THE  PLIMPTON  PRESS  •  NORWOOD  •  MASS 


TO   MY  FRIEND 

EDWARD  DEAN  ADAMS 

THIS   BOOK   IS   AFFECTIONATELY   DEDICATED 


PREFACE 

THIS  book  owes  its  inception  to  the  keen  scientific 
interest  of  Mr.  Edward  D.  Adams.  The  magnifi- 
cent painting  of  the  corona  by  Howard  Russell 
Butler,  which  now  finds  an  honored  place  in  the  American 
Museum  of  Natural  History,  New  York,  was  originally 
planned  that  it  might  provide  a  frontispiece  illustration. 

Fortunately  for  the  author,  he  had  the  hearty  coopera- 
tion of  many  of  his  friends  in  the  preparation  of  the  book. 
Professor  E.  E.  Barnard,  an  intimate  friend  for  twenty-five 
years,  furnished  all  of  the  photographs  illustrating  the 
eclipse  expedition  in  1901  to  Sumatra.  The  last  work 
undertaken  by  him,  a  few  days  before  his  lamented  death, 
was  to  dictate  the  captions  to  accompany  each  of  the  photo- 
graphs. Another  life-long  friend,  Professor  Edwin  B.  Frost, 
director  of  the  Yerkes  Observatory,  was  most  generous  in 
his  assistance.  As  editor  of  the  Astrophysical  Journal,  he  was 
instrumental  in  providing  the  blocks  for  many  of  the  illustra- 
tions. Professor  George  B.  Pegram,  chairman  of  the  Adams 
Fellowship  Committee  of  Columbia  University,  showed  con- 
tinued interest  during  the  progress  of  the  work.  Dr.  W.  W. 
Campbell,  director  of  the  Lick  Observatory,  kindly  furnished 
information  in  advance  of  publication  regarding  the  Einstein 
problem  and  provided  photographs  of  the  1922  eclipse  and 
with  great  generosity  supplied  many  additional  illustrations 
from  the  incomparable  Lick  series  of  photographs  of  the  solar 
corona,  while  Professor  H.  Deslandres  kindly  sent  for  repro- 
duction many  photographs  taken  with  the  spectro-heliograph 
at  Meudon. 

The  author  wishes  to  express  to  these  scientists  his  heart- 
felt appreciation  for  their  assistance  so  freely  given. 

The  second  edition  was  prepared  while  the  author  was 
in  residence  for  a  brief  stay  at  the  Mount  Wilson  Observa- 
tory. 

In  the  eight  years  which  separated  the  second  and  third  edi- 


viii  PREFACE 

tions,  scientists  had  a  most  thrilling  time  in  their  investiga- 
tions of  the  chemical  atom.  It  has  been  said  with  truth  that 
discoveries  have  followed  each  other  in  such  rapid  succession 
that  before  the  printer's  ink  was  dry,  a  new  and,  at  times, 
startling  theory  was  superseded  by  another  theory  still  newer. 

As  the  problems  of  solar  physics  have  their  fundamental 
basis  in  the  structure  of  the  atom,  an  attempt  has  been  made 
to  incorporate  in  this  book  the  recent  important  researches 
dealing  with  atomic  physics. 

The  fourth  edition,  prepared  while  in  residence  again  at 
Mount  Wilson  Observatory,  adds  to  the  third  edition  a  chap- 
ter giving  the  results  obtained  from  the  1932  and  1934 
eclipses.  Minor  changes  have  been  made  in  Chapter  IV  deal- 
ing with  future  eclipses. 

S.  A.  MITCHELL 

LEANDER  McCoRMicK  OBSERVATORY 
May  13, 1935 


CONTENTS 

INTRODUCTION xiii-xviii 

I.    EARLY  HISTORICAL  ECLIPSES    .  1-26 

CHINESE  ECLIPSES  —  ECLIPSES  OF  BABYLON  —  SIGNS  OF  THE 
ZODIAC  —  EGYPTIAN  ASTRONOMY 

II.    BIBLICAL  AND  CLASSICAL  ECLIPSES 27-36 

DATES  OF  THE  BIBLE  —  ECLIPSES  OF  CLASSICAL  LITERATURE  — 
ECLIPSE  OF  THALES  —  XERXES 

III.  THE  PREDICTION  OF  ECLIPSES 37-52 

METONIC  CYCLE  —  ARISTOTLE  —  HIPPARCHUS  —  SAROS  — 
THEORY  OF  ECLIPSES  —  ECLIPTIC  LIMITS 

IV.  THE  VERIFICATION  OF  ECLIPSES 53-71 

LONG-RANGE  PREDICTIONS  —  ONE  HUNDRED  YEARS  OF  TOTAL 
ECLIPSES  — NUMBER  OF  ECLIPSES  —  CALCULATION  OF  ECLIPSES 
—  LUNAR  AND  SOLAR  ECLIPSES  —  TABLES  OF  THE  MOON  — 
MOON  AHEAD  OF  PREDICTED  PLACE 

V.    THE  SPECTROSCOPE 72-90 

NEWTON  —  WOLLASTON  —  FRAUNHOFER  —  PRISMS  AND  GRAT- 
INGS —  WAVE-LENGTHS  —  FRAUNHOFER  LINES 

VI.   THE  SPECTROSCOPE  (CONTINUED) 91-105 

KlRCHHOFF  —  KlRCHHOFF  LAWS  —  ROWLAND  GRATINGS 

VII.   THE  SURFACE  OF  THE  SUN 106-127 

GREEK  IDEAS  —  DISTANCE  OF  THE  SUN  —  SIZE  OF  SUN  — 
PHOTOSPHERE  —  JANSSEN  —  SUN-SPOTS  —  PERIODICITY  OF 
SPOTS  — -  Is  THE  SUN  SPHERICAL? 

VIII.    MODERN  ECLIPSES  BEFORE  1878 128-150 

TYCHO  BRAKE  —  BAILY'S  BEADS  —  ECLIPSE  OF  1842  —  AT 
ECLIPSE  OF  1860,  PROMINENCES  DISCOVERED  TO  BE  SOLAR 
IN  ORIGIN  —  TRIUMPH  OF  THE  SPECTROSCOPE  AT  ECLIPSE  OF 
1868 

IX.    NINETEENTH  CENTURY  ECLIPSES  AFTER  1878    ....   151-166 
GREAT  EXTENSIONS  OF  THE  CORONA  IN  1878  —  INTRA-MERCU- 
RIAL  PLANETS  —  SPECTRUM  OF  THE  CORONA  —  FLASH  SPECTRUM 
DISCOVERED  IN   1870  —  IMPORTANCE  OF  PHOTOGRAPHY  — 
RESULTS  OF  1900 

X.    PERSONAL  EXPERIENCES  IN  1900  AND  1901 167-182 

ECLIPSE  OF  1900  —  To  SUMATRA  IN  1901  —  SCIENTIFIC  RESULTS 

XL   THE  SPANISH  ECLIPSE  OF  1905 183-190 

PHOTOGRAPHING  THE  FLASH  SPECTRUM 

XII.    THE  AMERICAN  ECLIPSE  OF  1918 191-202 

THREATENING  WEATHER  —  PAINTING  THE  CORONA 

XIII.   ECLIPSES  SINCE  1923 203-225 

EXPERIENCES  IN  CALIFORNIA  IN  1923  —  IN  CONNECTICUT  IN 
1925  — IN  NORWAY  IN  1927  —  ON  NIUAFOOU  ISLAND  IN  1930 


CONTENTS 


XIV,   THE  STRUCTURE  OF  THE  ATOM 226-246 

SIZE  OF  UNIVERSE  CONTRASTED  WITH  THE  SIZE  OF  THE  ELECTRON 

—  RADIUM  AND  ITS  RAYS  —  THE  ELECTRON  —  ATOMIC  NUM- 
BERS —  ISOTOPES  —  QUANTUM  THEORY  —  SPECTRAL  SERIES  — 
THE  BOHR  ATOM  —  SPECTRUM  OF  HYDROGEN  AND  HELIUM 

XV.    PHOTOGRAPHING  THE  FLASH  SPECTRUM 247-276 

CHOICE  OF  BEST  TYPE  OF  SPECTROGRAPH  —  SLIT  VERSUS  SLITLESS 

—  PRISMS  VERSUS  GRATINGS  —  FIXED  VERSUS  MOVING  PLATES 

XVI.    DISCUSSING  THE  FLASH  SPECTRUM 277-298 

CHROMOSPHERIC  INTENSITIES  ESTIMATED  AND  MEASURED  —  RE- 
SULTS OF  PANNEKOEK  AND  MINNAKRT  AND  OF  MENZEL  —  FLASH 
SPECTRUM  OBSERVED  AT  PARTIAL  ECLIPSES  AND  WITHOUT  AN 
ECLIPSE  —  PRESENT  PROBLEMS 

XVII.   THE  IMPORTANCE  OF  IONIZATION 299-318 

SAHA'S  THEORY  APPLIED  TO  THE  SPKCTRA  OF  SUN,  CHROMO- 
SPHERE, SUN-SPOTS  AND  STARS  —  PERIODIC  TABLE  OF  ELE- 
MENTS —  ATOMIC  THEORY  —  IONIZATION  AND  EXCITATION 
POTENTIALS  —  MULTIPLET  GROUPS 

XVIII.   HEIGHTS  AND  RELATED  PROBLEMS 319-337 

SOLAR  ROTATION  —  SUN-SPOT  VORTICES  —  POLARITIES  —  SUN- 
SPOT  PERIOD 

XIX.   HEIGHTS  AND  DISTRIBUTIONS  OF  VAPORS 338-359 

MULTIPLET  GROUPS  IN  IRON  —  EVERSHED  EFFECT  IN  SUN-SPOTS 

—  CORRELATIONS  BETWEEN  HEIGHTS,  INTENSITIES,  EXCITA- 
TION POTENTIALS,  EVERSHED  EFFECT  AND  RELATIVITY  SHIFT  — 
ABUNDANCES  AND  DENSITY  DISTRIBUTION 

XX.   THE  CORONA 360-386 

PAUCITY  OF  TIME  ALLOWED  FOR  INVESTIGATIONS  —  MOTIONS 
WITHIN  THE  CORONA  —  DISTURBANCES  —  WAVE-LENGTHS  — 
INTENSITIES  —  CORONIUM  —  ROTATION  OF  CORONA  —  UNSAT- 
ISFACTORY NATURE  OF  KNOWLEDGE 

XXI.   CORONAL  THEORIES 387-420 

POLARIZATION  —  LAW  OF  DISTRIBUTION  OF  LIGHT  WITHIN  THE 
CORONA  —  TOTAL  LIGHT  OF  CORONA  —  SHAPE  OF  CORONA  — 
CONNECTION  WITH  SUN-SPOT  PERIOD  —  VARIOUS  THEORIES 

XXII.   THE  EINSTEIN  THEORY  OF  RELATIVITY 421-442 

HAS  NEWTON'S  LAW  OF  GRAVITATION  BEEN  DISCARDED?  —  SPACE 
OF  FOUR  DIMENSIONS  —  MICHELSON-MORLEY  EXPERIMENT  — 
RELATIVITY  OF  MOTION  —  WHAT  Is  A  STRAIGHT  LINE?  — 
EVENTS  —  CONSEQUENCES  OF  EINSTEIN'S  THEORY 

XXIII.  HAS  THE  EINSTEIN  THEORY  BEEN  VERIFIED?  ....  443-479 

ECLIPSE  OF  1919  —  RESULTS  FROM  THE  ECLIPSES  OF  1922  AND 
1929  —  THE  SPECTROSCOPIC  TEST  —  GENERAL  CONCLUSIONS 

XXIV.  SHADOW  BANDS 480-482 

XXV.   THE  ECLIPSES  OF  1932  AND  1934 483-511 

NEW  INFORMATION  ABOUT  THE  CORONA  —  ECLIPSE  PROBLEMS 

INDEX 513-520 


ILLUSTRATIONS 

TOTAL  ECLIPSE  OF  1923,  IN  COLOR  (Butler) Frontispiece 

CORONAL  DISTURBANCES  AT  1930  ECLIPSE  (Marriott) 8 

CORONA  AT  1930  ECLIPSE  (Marriott) 9 

INNER  CORONA,  1930  (Marriott) 12 

THE  CORONA,   1929    (Swarthmore) 13 

AN   ANCIENT   EGYPTIAN   CALENDAR 24 

QUIESCENT  SOLAR  PROMINENCE 25 

FORTY-FOUR  ECLIPSES  IN  A  SERIES  FROM  1211  TO  1986 52 

SWARTHMORE  EXPEDITION  IN  1929,  HUGE  CAMERA 53 

TRACKS  OF  ECLIPSES  BETWEEN  1919  AND  1940 56 

THE  ECLIPSE  ON  NIUAFOOU  ISLAND 57 

TOTAL  ECLIPSE  OF  JUNE  8,  1918,  IN  COLOR  (Butler) 60 

TWENTY-FOUR-HOUR   DEVELOPMENT  OF   SUN-SPOT    (Mt.   Wilson) 84 

6o-FOOT  TOWER  TELESCOPE  AT  MT.  WILSON  OBSERVATORY 85 

THE  SUN  IN  THE  LIGHT  OF  GLOWING  CALCIUM  VAPOR 108 

SPECTROHELIOGRAMS  SHOWING  DEVELOPMENT  OF  SPOT 109 

THE  SUN'S  SURFACE,  SEPTEMBER  1883  (Janssen) 116 

SUN-SPOT  GROUP  (Meudon  Observatory) 117 

DETAILS  OF  PROMINENCES,  JUNE  8,  1918,  IN  COLOR  (Butler) 128 

OUTER  LAYERS  OF  THE  CHROMOSPHERE  (Deslandres) 136 

TEST  OF  RADIAL  MOTIONS  IN  SUN  (Deslandres) 137 

THE  CORONA  PHOTOGRAPHED  BY  LICK  PARTIES 148 

PROMINENCES  IN  PROJECTION  AGAINST  THE  SUN'S  DISK 149 

CORONA  OF  MAY  28,  1900  (Barnard  and  Ritchey) 164 

PROMINENCES  PHOTOGRAPHED  WITHOUT  AN  ECLIPSE  (Yerkes) 165 

PASIG  RIVER;  KRAKATOA  (Barnard) 168 

RAILROAD  IN  SUMATRA;  WATER  BUFFALO  (Barnard) 169 

SUMATRAN  BEAUTY;  MARKET  SCENE  (Barnard) 176 

SUMATRANS,  MALE  AND  FEMALE  (Barnard) 177 

FILIPINO  AND  SUMATRAN  HOMES  (Barnard) 184 

SCENES  AT  DAROCA,  SPAIN  (Barnard) 185 

THE  "  HELIOSAURUS  "AT  1918  ECLIPSE  (Barnard) 196 

AMERICAN  ECLIPSE  OF  1918,  LICK  PHOTOGRAPH 197 

MOUNT  WILSON  INSTRUMENTS  AT  1923  ECLIPSE 208 

DISTURBED  REGION  IN  1923  CORONA  (Swarthmore) 209 

CORONA  OF  1923  (Swarthmore) 212 

THE  CORONA  OF  JANUARY  24,  1925  (Slocum) 213 

THE  CORONA  OF  JANUARY  24,  1925,  IN  COLOR  (Butler) 224 

FLASH  SPECTRUM,  1905,  REGION  OF  H  AND  K  .    .   .          248 

FLASH  SPECTRUM  WITH  CONCAVE  GRATING  IN  BLUE  AND  GREEN  REGION    .  249 


xii  ILLUSTRATIONS 

PACE 

FLASH  SPECTRUM  WITH  MOVING  PLATE  (Campbell) 260 

CHANGES  IN  SOLAR  PROMINENCES  (Slocum) 261 

FRAUNHOFER  AND  FLASH  SPECTRA  COMPARED 268 

OBSERVERS  AT  THE  1905  ECLIPSE  IN  SPAIN 269 

PROMINENCE  OF  MAY  29,  1919,  PHOTOGRAPHED  WITHOUT  AN  ECLIPSE  ....  272 

TOTAL  ECLIPSE  OF  MAY  29,  1919 273 

APPROACHING  SHADOW  AT  ECLIPSE  OF  JUNE  1918,  IN  COLOR  (Butler)   ....  284 

CALCIUM  FLOCCULI  (Fox) 300 

ECLIPSE  CAMERAS  IN  1905  AND  1918 301 

SUN-SPOT  AND  PROMINENCE  (Mt.  Wilson) 320 

PROMINENCE  PROJECTED  ON  SUN'S  DISK  (Mt.  Wilson) 321 

ZEEMAN  EFFECT  IN  SUN-SPOTS  (Mt.  Wilson) 332 

BIPOLAR  SUN-SPOT  (Mt.  Wilson) 333 

SOLAR  VORTICES  (Mt.  Wilson) 340 

SPECTRUM  OF  SUN  AND  SPOT  COMPARED  (Mt.  Wilson) 341 

THE  SUN,  DIRECT  AND  IN  GLOWING  CALCIUM  VAPOR  (Yerkes) 348 

THE  TOTAL  ECLIPSE  OF  1918  (Lick) 349 

CORONA  BY  LICK  AND  CANADIAN  ASTRONOMERS 364 

TOTAL  ECLIPSE  OF  1922  (Lick) 365 

CORONAL  RINGS  AT  1930  ECLIPSE 372 

DETAILED  STRUCTURE  IN  CORONA,  1930 373 

LICK  PHOTOGRAPHS  OF  CORONAL  SPECTRUM,  1918 380 

GENERAL  SPECTRUM  OF  CORONA  (Lowell) 381 

CORONA  IN  POLARIZED  LIGHT,  1908  (Lick) 388 

CURVES  SHOWING  INTENSITY  IN  CORONA  (Bergstrand) 389 

INNER  CORONA  IN  1926  (Swarthmorc) 400 

LION-LIKE  PROMINENCE  AT  1929  ECLIPSE  (Swarthmore) 401 

EINSTEIN  CAMERAS  AT  THE  1919  ECLIPSE 424 

ECLIPSE  OF  1922,  CANADIAN  PHOTOGRAPHS 425 

CORONA  IN  1922   (Lick) 448 

FINE  STRUCTURE  IN  CORONA,  1929  (Swarthmore) 449 

LICK  CAMERA  FOR  TESTING  THE  EINSTEIN  DEFLECTION 460 

AUSTRALIAN  SAILORS  ERECTING  CANADIAN  CAMERAS  INI922 461 

CORONA  IN  1932   (Lick) 476 

EINSTEIN  CAMERA  IN  1934 477 

CLOUDS  AND  PROMINENCES  IN  1932 484 

MOON'S  SHADOW  AT  FIVE-SECOND  INTERVALS 485 

FLASH  SPECTRUM  IN  1932  (Greenwich) 492 

FLASH  SPECTRUM  IN  1932  (Lick) 493 

CORONA  IN  1934  (Satome) 500 

INSTRUMENTS  AT  THE  1934  ECLIPSE 501 


INTRODUCTION 

SCIENCE  seems  bent  on  increasing  man's  estimate  of 
the  span  of  time  since  creation  began.  It  is  now 
confidently  believed  that  the  human  race  has  in- 
habited this  terrestrial  ball  for  at  least  five  hundred  thousand 
years,  while  Mother  Earth  herself  must  have  been  in  exist- 
ence for  a  thousand  million  years.  On  the  assumption  that 
mass  is  electrical  and  that  all  kinds  of  energy  possess  mass, 
we  have  found  that  the  sun's  heat  can  not  be  maintained 
for  a  space  of  time  exceeding  fifteen  millions  of  millions  of 
years,  provided  all  its  energy,  including  that  of  the  atom, 
could  be  liberated.  Surely  such  an  interval  is  sufficiently 
great  to  satisfy  even  the  most  exacting!  And  yet  what 
changes  have  taken  place  in  our  mode  of  life  within  the 
memory  of  man! 

Today,  as  never  before,  our  daily  life  follows  its  course 
surrounded  by  the  wonders  of  science.  The  age  of  steam  has 
brought  the  powerful  locomotive  drawing  its  fast  and  com- 
fortable passenger  train  on  its  long  journey  across  the  con- 
tinent, has  constructed  the  gigantic  ocean  steamship  where- 
by Europe  is  brought  within  four  days  of  the  shores  of 
America,  has  been,  indeed,  the  cause  of  the  remarkable  de- 
velopment experienced  in  all  quarters  of  the  globe.  The 
internal  combustion  engine  has  largely  revolutionized  modes 
3f  transportation  on  the  land,  on  the  surface  of  and  under 
the  sea  and  in  the  air,  so  that  the  extravagantly  fanciful 
writings  of  Jules  Verne  are  common-place,  every-day  reali- 
ties. The  age  of  electricity  has  wrought  the  greatest 
:hanges  of  all  in  our  daily  life  bringing  as  it  has  stupendous 
transformations  in  illumination  vastly  different  from  the 
Feeble  tallow  dip;  it  has  wrought  the  powerful  motor;  it 
las  made  possible  the  electric  telegraph  and  telephone  and 
:he  still  more  wonderful  wireless,  with  the  result  that  dis- 
;ance  is  eliminated  and  there  is  served  fresh  each  morning 

xiii 


xiv  INTRODUCTION 

with  our  coffee  the  very  latest  news  garnered  from  every 
quarter  of  the  globe.  Through  the  agency  of  the  cinema 
and  the  wireless  telephone  the  inhabitant  of  a  backwoods 
village  can  gaze  upon  great  actors,  can  listen  to  the  greatest 
of  singers  and  the  best  of  symphony  orchestras  quite  as  well 
as  if  he  lived  in  London,  Paris  or  New  York. 

The  bridge  separating  pure  from  applied  science  has  been 
so  much  shortened  that  the  gap  has  practically  disappeared. 
The  refined  researches  of  the  physicist  or  chemist  today 
may  be  applied  in  the  arts  and  sciences  of  tomorrow.  In- 
vestigations in  Hertzian  waves  in  the  laboratory  have  made 
the  wireless  telephone  possible,  the  discovery  of  Mme.  Curie 
and  the  remarkable  work  of  Rutherford  and  others  on  the 
penetrating  powers  of  the  rays  of  radium  have  given  to  the 
cancer  specialist  the  means  of  alleviating  the  horrors  of  this 
dread  disease ;  X-rays  are  now  in  every-day  use  by  the  doc- 
tor, the  surgeon  and  the  dentist.  And  in  the  quiet  rest  of 
the  laboratory,  the  chemist  has  experimented  with  his  vials 
and  test  tubes  and  has  made  noxious  and  poisonous  gases 
which,  liberated  amidst  the  din  and  roar  of  the  battlefield, 
have  caused  the  destruction  of  thousands  of  lives. 

It  is  less  than  forty  years  since  the  discovery  of  X-rays 
and  radium.  The  marvelous  researches  connected  with 
radioactivity  have  shown  the  amazing  degree  of  refinement 
necessary  in  the  work  of  the  scientist.  New  forces,  hitherto 
unknown,  have  been  discovered,  while  recent  researches 
display  the  amazing  fact  that  each  chemical  atom  takes 
its  place  in  Nature  as  a  miniature  solar  system.  Modern 
investigations  have  revealed  that  one  chemical  element  may 
be  transformed  into  another,  thus  realizing  the  age-long 
dream  of  the  alchemist,  the  transmutation  of  one  metal  into 
another.  It  has  been  proved  that  several  separate  series  of 
radioactive  transformations  have  as  their  end-product  the 
metal  lead  —  but  fortunately  for  the  economic  future  of  the 
world,  the  realization  of  the  alchemist's  vision  of  changing 
base  lead  into  glittering  gold  seems  as  far  distant  as  ever. 
Helium  is  a  product  of  almost  every  radioactive  transforma- 
tion and  it  has  thus  become  a  household  word  with  every 
trained  physicist.  And  yet  helium  was  discovered  —  a  re- 


INTRODUCTION  xv 

suit  of  solar  eclipse  observations  —  a  bare  half-century  ago 
and  was  isolated  from  pitchblende  only  in  1895!  Within 
two  decades  the  demands  of  the  Great  War  brought  to  the 
attention  of  scientists  the  urgent  necessity  of  obviating  the 
disastrous  explosions  that  have  taken  place  with  hydrogen- 
filled  balloons  with  the  consequent  result  that  helium  was 
produced  in  such  large  quantities  that  balloons  were  filled 
with  this  inert  gas. 

But  amongst  all  the  wonders  of  all  the  wonderful  sciences 
there  is  no  science  which  deals  with  such  a  gorgeous  spec- 
tacle as  is  exhibited  by  the  queen  of  the  sciences,  astronomy, 
at  the  moment  when  the  earth  is  gradually  shrouded  in  dark- 
ness and  when  around  the  smiling  orb  of  day  there  appears 
the  matchless  crown  of  glory,  the  so-called  corona.  Nor  can 
any  science  duplicate  the  wonderful  precision  shown  by  the 
work  of  the  astronomer  in  his  capacity  to  predict  hundreds 
of  years  in  advance  the  exact  hour  and  minute  at  which  an 
eclipse  will  take  place  and  the  locality  on  the  earth's  surface 
where  such  an  eclipse  will  be  visible. 

The  great  progress  of  science  in  the  last  fifty  years  is 
nowhere  better  illustrated  than  in  the  attitude  of  astrono- 
mers towards  observations  at  the  time  of  a  total  eclipse  of 
the  sun.  Until  about  the  middle  of  the  nineteenth  century 
little  interest  was  taken  in  the  subject  although  information 
regarding  the  sun  was  comparatively  scant.  The  eclipse 
was  observed  only  if  perchance  its  track  happened  to  cross 
the  home  of  the  observer;  the  only  observations  of  value 
being  the  exact  times  of  contact  of  the  limbs  of  the  sun  and 
moon,  taken  for  the  purpose  of  perfecting  the  lunar  tables. 
The  beautiful  corona  was  watched  with  awe  and  admiration, 
a  few  sketches  were  made  of  its  form,  —  but  these  were 
done  with  such  indifferent  skill  that  they  added  but  little 
to  the  information  available  at  the  time.  Practically  no 
expeditions  were  equipped  and  sent  away  from  home.  How 
different  it  is  in  the  twentieth  century!  In  the  year*  1901, 
the  United  States  Naval  Observatory  financed  and  sent  an 
expedition  as  far  from  home  as  it  could  possibly  go  —  half 
way  round  the  world  —  and  all  for  the  purpose  of  making 
observations  during  a  few  short  minutes  of  time.  At  the 


xvi  INTRODUCTION 

recurrence  of  the  eclipse  in  1919,  the  British  observers  in 
Brazil  and  West  Africa  startled  the  thinking  world  by  veri- 
fication of  the  Einstein  problem.  In  September,  1922,  the 
British  astronomers  had  spent  more  than  six  months  on 
Christmas  Island  in  the  Indian  Ocean  only  to  have  their  work 
come  to  naught  because  of  clouds  at  the  critical  moment, 
while  American  and  Canadian  scientists  had  gone  to  North- 
west Australia  in  quest  of  information  about  the  relativity 
shift.  On  isolated  "  Tin-Can  Island  "  in  the  South  Pacific, 
important  discoveries  were  made  in  1930. 

To  make  such  expensive  and  time-consuming  expeditions 
worth  while  there  must  be  problems  connected  with  the  sun 
whose  solution  is  of  vital  importance  to  the  astronomer.  In 
this  book  the  author  will  endeavor  to  state  some  of  these 
problems  and  the  methods  devised  for  their  solution.  At 
the  same  time  it  will  be  necessary  to  refute  one  of  the 
"  twice-told  tales  "  (See  Science  during  1921)  that  the  moon 
is  a  more  important  body  than  the  sun  since  the  moon  gives 
us  light  at  night  when  it  is  dark  and  we  need  its  light, 
whereas  the  sun  shines  during  the  daytime  when  it  is  bright 
and  we  could  possibly  get  along  very  well  without  it!  The 
light  that  lightens  the  world,  the  heat  that  gives  our  bodies 
comfort,  the  wind  that  cools  our  heated  brows  and  wafts  the 
sailing  ships  across  the  ocean,  the  coal  that  warms  and  illumi- 
nates our  homes  and  generates  steam  and  electricity  to  carry 
on  the  world's  mighty  commerce,  the  rain  that  descends 
from  heaven  and  waters  and  fertilizes  the  soil  and  causes 
the  flow  of  our  mighty  rivers;  all  these  and  many  other 
benefits  come  from  the  sun.  Without  the  sun  there  would 
be  no  grass,  no  flowers,  no  wheat  or  corn  or  vegetable  life 
of  any  kind,  and  without  the  sun  there  would  be  no  animal 
life,  no  man  upon  the  earth.  If  the  sun  were  blotted  out 
for  the  space  of  one  short  month,  there  would  not  be  one  of 
us  left  alive  to  tell  the  tale;  we  should  all  be  frozen  to  death! 
It  is  well  then  that  we  should  endeavor  to  learn  as  much  as 
possible  of  the  giant  luminary  in  which  are  centered  our  light 
and  heat  and  our  very  life,  even  when  the  quest  for  knowl- 
edge carries  one  as  far  from  home  as  does  eclipse  work. 

The  life  of  such  an  observer  might  be  likened  to  that  of 


INTRODUCTION  xvii 

a  hunter  after  big  game.  Many  months  and  even  years  are 
spent  in  quietly  investigating  the  problems,  a  costly  equip- 
ment is  accumulated  and  each  piece  of  delicate  apparatus 
is  carefully  tested  at  home  to  see  that  it  will  properly  per- 
form its  designated  functions  far  afield,  a  long  journey  is 
often  necessary,  frequently  of  thousands  of  miles,  by  rail 
and  sea.  Arrived  at  the  destination,  instruments,  cameras 
and  spectroscopes  are  erected  and  most  carefully  adjusted, 
and  after  six  or  eight  weeks  of  preparation  in  the  field  the 
eventful  day  approaches.  Each  and  everyone  of  the  party 
drills  constantly  so  that  the  task  allotted  to  him  may  be  well 
and  carefully  done,  so  that  the  photographic  slides  may  be 
drawn  and  each  camera  lens  may  be  opened  at  the  appro- 
priate instant.  Success  lies  in  seeing  that  every  one  of  a 
thousand  possible  chances  of  failure  are  obviated.  At  a  cer- 
tain hour,  minute  and  second,  the  "  zero-hour,"  operations 
are  due  to  begin.  But  alas!  there  may  be  no  "  game,"  the 
eclipse  may  be  eclipsed  by  clouds,  and  the  long  months  of 
preparation  may  be  of  no  avail  since  it  will  not  be  possible 
to  try  again  on  the  morrow  when  the  clouds  have  rolled 
away. 

The  author  has  traveled  ninety  thousand  miles  to  witness 
nine  total  eclipses  of  the  sun.  The  total  accumulation  of  time 
afforded  him  for  scientific  observations  during  these  nine 
eclipses  has  been  approximately  eighteen  minutes. 


ECLIPSES  OF  THE  SUN 

CHAPTER   I 
EARLY  HISTORICAL  ECLIPSES 

"In  old  Cathay,  in  far  Cathay, 

Before  the  western  world  began, 
They  saw  the  moving  font  of  day 

Eclipsed,  as  by  a  shadowy  fan; 
They  stood  upon  their  Chinese  wall. 

They  saw  his  fire  to  ashes  fade, 
And  felt  the  deeper  slumber  fall 

On  domes  of  pearl  and  towers  of  jade."  —  NOYES. 

THE  earliest  recorded  eclipse  of  the  sun  is  one  which 
happened  more  than  four  thousand  years  ago, 
an  account  of  which  is  given  in  the  ancient  Chinese 
classic  Shu  Ching.  According  to  the  competent  authority 
Oppolzer,  this  took  place  on  October  22,  2137  B.C.,  about 
1400  years  more  remote  than  that  recorded  by  any  other 
nation.  This  eclipse  is  celebrated  not  only  -for  its  great 
antiquity,  but  also  for  the  dire  fate  of  the  two  royal  astron- 
omers, Hsi  and  Ho,  who  instead  of  staying  in  the  sober 
paths  of  science  went  and  got  beastly  drunk,  with  the  re- 
sult that  they  were  taken  unawares  and  were  unprepared 
to  perform  their  customary  rites  of  shooting  arrows,  beat- 
ing drums,  etc.,  for  the  purpose  of  delivering  the  sun  from 
the  monster  that  was  devouring  it.  To  show  his  great  dis- 
pleasure, not  so  much  for  failing  to  predict  the  eclipse,  but 
on  account  of  the  intense  confusion  that  prevailed,  Chung 
K'ang,  the  fourth  emperor  of  the  Hsai  dynasty,  ordered 
that  they  should  be  punished  and  that  their  heads  should 
be  chopped  off.  And  with  this  tragic  warning  in  view,  there 
is  no  record  from  that  day  to  this  that  an  astronomer  has 
ever  dared  to  follow  in  the  steps  of  the  unfortunates,  Hsi 
and  Ho;  and  been  drunk  at  the  time  of  an  eclipse. 


2  ECLIPSES    OF    THE    SUN 

This  eclipse  is  of  such  great  importance  that  it  will  be 
well  to  give  it  more  than  a  passing  glance.  The  early 
eclipses,  both  of  sun  and  of  moon,  have  been  of  such  great 
interest  to  astronomers  and  scholars  that  a  very  large  num- 
ber of  investigations  have  been  devoted  to  the  determina- 
tion of  as  exact  dates  as  possible  for  these  phenomena. 
Eclipses  can  take  place  only  when  the  centers  of  sun,  earth 
and  moon  are  approximately  in  a  straight  line.  These  cir- 
cumstances can  be  predicted  with  accuracy  if  we  know  with 
a  sufficient  degree  of  precision  the  motions  of  the  earth 
about  the  sun,  and  of  the  moon  about  the  earth.  The  times 
of  eclipses  thus  give  to  the  astronomer  the  means  of  ac- 
curately testing  his  calculations  regarding  the  motions  of 
both  these  heavenly  bodies.  The  movements  of  the  earth 
render  no  difficulties,  but  with  the  earth's  child,  the  moon, 
the  matter  is  entirely  different.  It  may  be  truly  said  that 
the  motions  of  the  moon  have  given  the  mathematical  as- 
tronomer more  work  and  worry  than  those  of  all  the  balance 
of  the  gigantic  universe  put  together.  The  earlier  the  eclipse 
that  can  be  checked  up  with  accuracy  the  more  reliably  can 
the  motion  of  the  moon  be  verified,  since  any  accelerations 
in  this  motion  depend  on  the  square  of  the  elapsed  time. 
Valuable  as  these  early  eclipses  are  to  the  astronomer,  they 
are  equally  important  to  the  historian  or  chronicler  in  fix- 
ing the  dates  of  remote  antiquity.  The  astronomical  prob- 
lem has  been  well  studied  and  is  now  regarded  as  a  simple 
one.  The  crux  of  the  whole  question  lies  in  the  exactness 
and  precision  with  which  the  event  has  been  described  by 
the  author  or  historian. 

The  Shu  Ching,  or  Book  of  Historical  Documents,  is  a 
collection  of  public  speeches  and  proclamations  beginning 
with  the  reign  of  the  legendary  Emperor  Yao  who  lived  in 
the  twenty-fourth  century  B.C.,  and  closing  with  the  year 
B.C.  625.  The  book  is  not  a  historical,  chronological  narra- 
tive, nor  indeed  does  such  a  book  exist  in  the  Chinese  lan- 
guage. This  ancient  eclipse  has  been  fully  discussed  by  a 
number  of  eminent  authorities.  The  most  complete  mono- 
graph is  by  Schlegel  and  Kiihnert,  Die  Shu-King-Finsterniss, 
Verhand.  der  Konin.  Akad.  van  Wettenschappen,  Letter- 


EARLY    HISTORICAL    ECLIPSES  3 

kunde  19,  5,  1890.  Oppolzer  communicated  to  Monats- 
berichte  der  Kbn.  Preuss.  Akad.  der  Wissens.  zu  Berlin,  166, 
1880.  Briefer  accounts  may  be  found  by  S.  M.  Russell, 
Observatory,  18,  323,  1895;  in  Chambers'  The  Story  of 
Eclipses,  65,  1900;  and  in  the  Halley  Lecture  delivered  by 
Dr.  J.  K.  Fotheringham,  May  17,  1921. 

In  the  third  century  B.C.,  Shi  Hwang- Yi,  a  great  military 
genius,  conquered  the  whole  of  the  China  of  those  days. 
In  221  B.C.  he  proclaimed  himself  the  "  First  Emperor,"  and 
he  decided,  on  the  advice  of  his  prime  minister,  that  every 
form  of  human  progress  including  literature  should  begin 
with  his  reign.  He  therefore  ordered  the  destruction  of  all 
books  except  those  belonging  to  the  three  utilitarian 
branches  of  knowledge,  agriculture,  divination  and  medicine. 
To  see  that  his  orders  were  properly  carried  out,  he  per- 
sonally examined  each  day  a  hundred  pounds  of  books  so 
that  he  might  decide  which  were  useless.  Four  hundred 
and  sixty  scholars  were  put  to  death  for  disobeying  the  royal 
commands,  and  others  were  banished  for  life.  All  copies 
of  the  Shu  Ching  seem  to  have  perished  except  one  incom- 
plete copy  later  recovered  from  a  receptacle  where  it  had 
been  walled  up  by  a  devoted  scholar.  The  book  in  which  the 
eclipse  is  mentioned  is  not  included  in  the  authentic  copy. 
It  was  added  later,  probably  to  conform  to  a  table  of  con- 
tents that  scholars  believe  may  have  been  included  in  the 
original  volume,  and  which  may  possibly  have  been  written 
by  the  great  Confucius.  The  preface  to  this  book  is  as 
follows: 

"  Hsi  and  Ho,  sunk  in  wine  and  excess,  neglected  the 
ordering  of  the  seasons,  and  allowed  the  days  to  get  into 
confusion.  The  prince  of  Yin  went  to  punish  them.  De- 
scription of  this  there  was  made,  The  Punitive  Expedition 
of  Yin." 

It  should  be  noted  in  the  above  that  there  is  no  mention 
of  an  eclipse.  Tso,  a  scholar  and  commentator  of  the  fifth 
century  before  Christ  wrote  concerning  the  lost  work,  The 
Punitive  Expedition  of  Yin  in  the  following  words,  "  The 
Sun  and  Moon  did  not  meet  harmoniously  in  Fang.  The 
blind  beat  their  drums;  the  inferior  officers  galloped  and 


4  ECLIPSES    OF    THE    SUN 

the  common  people  ran  about."  There  is  further  added, 
"  That  is  said  of  the  first  day  of  this  month;  it  was  in  the 
fourth  month  of  Hsia,  which  is  called  the  first  month  of 
summer." 

In  the  Annals  of  the  Bamboo  Books,  a  work  of  the  early 
third  century  before  Christ,  the  reference  to  Hsi  and  Ho  is 
as  follows:  "  In  the  fifth  year  of  Chung  K'ang,  in  the 
autumn,  in  the  ninth  month,  on  the  first  day  of  the  month, 
there  was  an  eclipse  of  the  Sun,  when  he  ordered  the  prince 
of  Yin  to  lead  the  imperial  forces  to  punish  Hsi  and  Ho." 

The  text  does  not  say  that  the  expedition  was  for  the 
purpose  of  punishing  the  astronomers  for  failing  to  predict 
the  eclipse.  The  scholar  who  restored  the  missing  text  used 
such  flowery  language  that  it  is  difficult  to  obtain  the  exact 
meaning,  although  it  seems  to  be  implied  that  the  eclipse 
was  the  cause  of  the  punitive  expedition. 

There  are  many  uncertainties  that  prevent  an  accurate 
interpretation  of  this  eclipse,  chief  among  which  may  be 
noted:  (i)  The  month  is  entirely  unknown,  whether  it  is 
the  "  first  day  of  the  last  month  of  autumn,"  or,  "  the  first 
day  of  the  first  month  of  summer."  (2)  The  meaning  of 
the  word  Fang  is  doubtful,  whether  it  means  "  the  order 
of  the  constellations,"  which  seems  to  be  the  better  opinion, 
or  whether  Fang  is  used  in  the  more  restricted  sense  as  the 
name  of  a  small  constellation  including  /3,  6,  TT  and  p 
Scorpii.  (3)  The  Emperor's  name  is  very  uncertain  as  it  is 
not  given  except  in  the  Bamboo  Books.  Little  is  actually 
known  of  Chinese  chronology  further  back  than  B.C.  841. 
We  do  know  the  names  of  the  emperors  in  succession,  but 
whether  they  reigned  five  years  or  fifty  is  unknown,  and 
consequently  all  that  it  is  possible  to  say  about  the  emperor 
Chung  K'ang  is  that  he  must  have  been  on  the  throne  a 
century  or  two  earlier  or  later  than  the  year  2000  B.C. 
(According  to  the  Encyclopaedia  Britannica  the  first  of  the 
kings  mentioned  in  the  Shu  Ching  reigned  from  2357  to 
2255  B-c->  an  interval  of  no  less  than  102  years!) 

One  interpretation  of  the  above  uncertainties  is  probably 
about  as  good  as  another.  One  date  fixed  by  the  Chinese 
astronomers  of  about  the  eighth  century  A.D.,  was  the  year 


EARLY    HISTORICAL    ECLIPSES  5 

B.C.  2155.  But  in  consequence  of  the  fact  that  the  eclipse 
of  that  year  was  invisible  in  China,  there  has  arisen  the 
well-known  ditty: 

"  Here  lie  the  bones  of  Ho  and  Hi, 
Whose  fate  though  sad  was  risible, 
Being  hanged  because  they  could  not  spy 
The  eclipse  which  was  invisible." 

Taking  "  the  last  month  of  Autumn,"  and  Fang  in  the 
restricted  meaning  of  the  word  that  it  referred  to  the  con- 
stellation, Oppolzer  fixes  the  date  as  given  above,  B.C.  2137, 
October  22.  Other  authorities  equally  competent  give  other 
dates.  The  only  conclusion  that  can  be  reasonably  drawn 
is  that  it  is  not  possible  to  identify  the  eclipse  with  any 
approximation  to  certainty.  Nothing  is  known  of  the  fates 
of  the  astronomers  Hsi  and  Ho.  The  regulations  regarding 
eclipses  as  given  in  the  Shu  King  read:  "  Being  before  the 
time,  the  astronomers  are  to  be  killed  without  respite;  and 
being  behind  the  time,  they  are  to  be  slain  without  re- 
prieve." With  such  a  fate  in  store,  who  would  dare  be  an 
astronomer? 

Many  admirers  of  the  Chinese,  not  being  conversant  with 
the  true  facts,  have  pointed  out  this  eclipse  as  evidence  of 
the  great  learning  of  the  Chinese  and  their  proficiency  in 
astronomical  knowledge  twenty  centuries  before  the  be- 
ginning of  the  Christian  era.  Although  not  wishing  to  dim 
the  halo  of  glory  that  has  surrounded  early  China,  honesty 
compels  one  to  draw  attention  to  the  facts.  For  many 
centuries  the  Chinese  were  unable  to  predict  the  position  of 
the  sun  accurately  among  the  stars  for  determining  the 
length  of  the  year.  In  fact,  they  seem  to  have  relied  wholly 
on  observation  from  year  to  year  to  settle  their  calendar. 
No  conclusions  or  systematic  deductions  were  made  from 
their  observations  as  is  shown  by  the  fact  that  their  calendar 
was  continually  falling  into  confusion. 

To  those  who  are  interested  in  early  chronological  data  to 
be  fixed  by  this  and  other  remote  eclipses,  reference  should 
be  directed  also  to  Delambre,  Histoire  de  I' Astronomic 
Ancienne,  Paris  1817,  and  to  Johnson's  Historical  and 
Future  Eclipses,  London,  1896. 


6  ECLIPSES    OF    THE    SUN 

OTHER  CHINESE  ECLIPSES 

The  history  of  the  Chinese  Empire  is  very  closely  asso- 
ciated with  the  name  of  Confucius,  the  immortal  sage. 
Confucius  is  the  Latinized  form  of  K'ung  tsze  (meaning 
the  philosopher  K'ung,  this  being  the  family  name). 
Through  his  veins  flowed  the  best  blood  of  China.  His 
birth  has  an  interesting  romance  connected  with  it.  His 
father  Heih  had  a  family  of  nine  daughters  but  only  one 
son  who  was  unfortunately  a  cripple.  Realizing  in  his  old 
age  that  the  K'ung  name  would  probably  become  extinct 
with  him,  he  went  to  his  neighbor  of  the  Yen  clan,  and  told 
of  his  plight,  and  asked  in  marriage  one  of  Yen's  three 
daughters.  The  youngest  of  the  three  became  the  mother 
of  Confucius,  551  or  550  B.C.,  Heih  then  being  over  seventy 
years  of  age.  The  father  died  in  the  child's  third  year. 

In  his  twenty-second  year  Confucius  began  his  life  as  a 
teacher,  and  he  put  into  practice  principles  which  every  col- 
lege professor  of  the  twentieth  century  would  like  to  follow, 
namely,  he  would  accept  any  pupil  no  matter  how  small  the 
fee,  but  as  soon  as  lack  of  capacity  or  diligence  was  mani- 
fested, the  youth  was  sent  away.  "  When  I  have  pre- 
sented," he  is  reported  to  have  said,  "  one  corner  of  a  subject 
and  the  pup\l  cannot  of  himself  make  out  the  other  three, 
I  do  not  repeat  my  lesson."  Would  that  these  methods  of 
education  were  prevalent  today!  His  professed  disciples 
numbered  3000,  of  whom  70  or  80  were  "  scholars  of 
extraordinary  ability."  Most  of  his  life  was  spent  in  the 
province  of  Lu,  the  modern  Ho-nan  and  Shantung. 

Among  the  writings  ascribed  to  Confucius  are  the  follow- 
ing: 

(1)  A  preface  to  the  Shu  Ching,  already  referred  to. 
The  preface  is  little  more  than  a  table  of  contents,  and  it  is 
very  doubtful  if  Confucius  had  anything  to  do  with  it. 

(2)  He  compiled,  or  edited  the  Shih  Ching,  or  Book  of 
Poems.     Originally  numbering  about  3000,  only  311  were 
retained  by  Confucius,  of  which  we  now  possess  305.     It  is 
the  most  ancient  book  of  poetry  in  the  world.    The  latest 
of  the  poems  has  the  date  585  B,C. 


EARLY    HISTORICAL    ECLIPSES  7 

(3)  From  his  own  hand  comes  the  Ch'un  CWin,  or 
"  Spring  and  Autumn/'  which  is  best  known  as  the  Annals 
of  Lu,  his  native  state.  This  work  has  been  preserved  al- 
most complete.  It  is  a  brief  summary  of  the  chief  events 
that  took  place  in  Lu  during  a  period  of  242  years  from 
722  to  481  B.C.  It  is  a  model  for  a  historical  document. 
The  facts  are  briefly  itemized  according  to  the  seasons  in 
which  they  fell.  As  an  example,  in  the  year  B.C.  612  there 
are  twelve  entries,  the  fifth  of  which  was  recorded,  "  In  the 
autumn,  in  the  seventh  month,  there  was  a  comet  which 
entered  Pei-ton  (in  Ursa  Major)." 

In  the  Annals  of  Lu  are  records  of  no  less  than  thirty- 
six  eclipses  of  the  sun,  the  first  eclipse  being  observed 
February  22,  720  B.C.,  and  the  last  on  July  22,  495  B.C. 
The  first  of  them  is  described  as  follows:  "  In  the  58th 
year  of  the  32nd  cycle  in  the  $ist  year  of  the  Emperor 
King-Wang  of  the  Chou  Dynasty,  the  3rd  year  of  Yin- 
Kung,  Prince  of  Lu,  in  the  spring,  the  second  moon,  on  the 
day  called  Ke-Sze,  there  was  an  eclipse  of  the  Sun." 
These  ancient  eclipses  have  been  carefully  investigated  and 
many  papers  concerning  them  have  been  published  in  the 
Monthly  Notices  of  the  Royal  Astronomical  Society.  The 
conclusions  are  that  it  has  been  possible  to  identify  no  less 
than  thirty-two  of  the  thirty-six.  Apparently  four  of  the 
eclipses,  those  of  April,  645  B.C.,  June,  592  B.C.,  September 
19,  552  B.C.,  and  June  18,  549  B.C.  are  in  error  and  did  not 
take  place.  The  record  is  a  history  of  eclipses  that  were  ac- 
tually observed  to  have  taken  place.  The  inference  is 
readily  drawn  that  the  astronomers  who  recorded  these  er- 
roneous eclipses  saw  some  curious  phenomenon  around  the 
sun;  and  remembering  the  sad  fate  that  befell  the  two 
negligent  astronomers,  Hsi  and  Ho,  and  not  wishing  to  take 
any  chances  whatever  regarding  the  safety  of  their  heads, 
they  recorded  the  phenomenon  as  an  eclipse. 

In  addition  to  the  works  from  the  hand  of  Confucius  him- 
self we  are  fortunately  in  possession  of  the  Tso  Chuan,  a  so- 
called  commentary.  This  is  presumably  by  some  one  with 
the  name  of  Tso  who  took  the  bare  entries  in  the  work 
by  Confucius  and  enlarged  upon  each  one  to  such  an  extent 


8  ECLIPSES    OF    THE    SUN 

and  with  such  remarkable  genius  and  dramatic  brilliancy 
that  the  Tso  commentary  reads  more  like  a  prose  epic  than 
an  elaboration  of  a  series  of  facts  or  annotations  on  the 
text  of  a  literary  work.  By  its  means  there  is  vividly  por- 
trayed the  intrigues,  the  alliances,  the  treacheries  and  the 
jealousies  of  the  various  states  which  made  up  feudal  China; 
we  can  see  with  the  clearness  almost  of  a  photograph,  as- 
sassinations, battles,  heroic  deeds,  brilliant  rescues  and  the 
torments  and  horrors  meted  out  by  the  conquerors  to  their 
unfortunate  victims.  The  Annals  of  Lu  make  the  chronicles 
of  China  in  the  sth,  6th  and  yth  centuries  before  the  birth 
of  Christ  as  full  of  action  and  as  attractive  as  those  of 
France  and  England  twenty  centuries  later.  As  a  matter 
of  fact  there  appeared  to  be  more  of  literary  culture  in 
China  in  these  early  centuries  B.C.  than  there  was  in  Europe 
a  bare  five  hundred  years  ago. 

The  eclipse  of  February  22,  720  B.C.  recorded  in  the 
Annals  of  Lu  is  not  the  first.  Chinese  eclipse  whose  date  can 
be  accurately  fixed,  for  fifty-six  years  earlier  there  was  an 
eclipse  of  the  sun  preceded  by  an  eclipse  of  the  moon.  In 
the  Shift  Ching,  or  Book  of  Poetry  referred  to  above,  there 
is  an  account  which  runs  somewhat  as  follows:  "  Tenth 
moon,  her  conjunction  first  day,  sin-mao,  sun  he  had  eclipse, 
also  it  very  bad."  The  record  ends  by  adding,  "  That  moon 
in  eclipse  is  a  thing  only  common.  This  sun  in  eclipse  is  a 
thing  very  bad."  The  eclipse  of  the  sun  referred  to  took 
place  on  September  6,  776  B.C.,  the  eclipse  of  the  moon 
on  August  2  r.  The  solar  eclipse  was  visible  only  as  a  partial 
eclipse  in  China  while  that  of  the  moon  was  nine-tenths 
total.  It  was  during  the  reign  of  the  notorius  Yu-wang. 
It  is  apparent  that  the  eclipse  of  the  sun  was  regarded  by 
the  bard  as  a  special  warning  to  the  lascivious  Emperor. 
The  evidence  of  this  eclipse  is  indisputable;  it  fixes* abso- 
lutely without  controversy  the  first  certain  date  in  <the 
history  of  China.  How  far  before  this  epoch  the  Chinese 
history  stretches  is  a  matter  of  personal  judgment  and  in- 
dividual conjecture.  As  already  pointed  out,  the  records 
of  early  dates  are  so  unreliable  that  one  of  the  early  kings 
is  supposed  to  have  reigned  the  impossible  period  of  102 


CORONAL  DISTURBANCES  TO  EAST  OF  SUN  IN  1930,  SHOWING  "  STRAWBERRY 

DOME  "  AND  LONGEST  STREAMER 
Photograph  by  Marriott  with  63 -foot  tower  telescope. 


EARLY    HISTORICAL    ECLIPSES  9 

years.  Egypt  has  her  pyramids  and  ruined  temples  as 
evidence  of  a  much  earlier  civilization,  but  there  are  no 
such  relics  in  China.  This  Chinese  eclipse  antedates  by 
thirteen  years  the  celebrated  Nineveh  eclipse. 

Mr.  John  Williams,  formerly  Assistant  Secretary  of  the 
Royal  Astronomical  Society,  made  a  careful  examination 
of  the  Chinese  historical  work  called  Tung-Keen-Kang- 
Muh,  a  record  of  one  hundred  and  one  volumes  which  con- 
tains a  summary  from  earliest  times  to  1368  A.D.  Informa- 
tion concerning  these  eclipses  can  be  found  in  the  Monthly 
Notices  R.  A.  S.  Fifty-six  solar  eclipses  occurred  between 
481  B.C.  and  the  beginning  of  the  Christian  era,  and  nearly 
one  hundred  additional  ones  before  the  fourth  century  A.D. 

ECLIPSES  OF  BABYLON 

"  In  Babylon,  in  Babylon, 

They  baked  their  tablets  of  the  clay; 
And,  year  by  year,  inscribed  thereon 

The  dark  eclipses  of  their  day; 
They  saw  the  moving  finger  write 

Its  Mene,  Mene,  on  their  sun. 
A  mightier  shadow  cloaks   their  light, 

And  clay  is  clay  in  Babylon."  —  NOYES. 

According  to  L.  W.  King,  Chronicles  concerning  the  early 
Babylonian  Kings,  2,  76,  1907,  it  is  recorded  that  "  on  the 
twenty-sixth  day  of  the  month  Sivan,  in  the  seventh  year, 
the  day  was  turned  to  night,  and  fire  in  the  midst  of 
heaven.  .  .  ."  Cowell *  has  concluded  that  this  probably  re- 
fers to  a  solar  eclipse  visible  in  Babylon  on  July  31,  1063  B-c- 
By  carrying  the  Babylonian  records  backwards  from  this 
eclipse,  a  check  is  had  on  the  acknowledged  Kea-Tsze 
6o-day  calendar  of  the  Chinese  from  which  comes  the  date 
of  the  eclipse  of  2137  B.C.  Unfortunately,  there  is  no 
knowledge  concerning  the  name  of  the  king  in  the  seventh 
year  of  whose  reign  the  eclipse  occurred.  If  this  fact  were 
known,  the  early  Babylonian  chronology  would  be  fixed 
with  fairly  great  certainty.  It  is  possible  to  conjecture  as 
Langdon  has  done  that  the  name  of  this  king  was  Nabu- 
shum-libur,  but  this  name  must  be  looked  upon  only  as  a 

1  Monthly  Notices,  R.  A.   S.,  65,   861,    1905. 


10  ECLIPSES    OF    THE    SUN 

suggestion.  All  that  is  known  of  exact  dates  in  Egyptian 
and  Babylonian  chronology  depends  on  the  science  of  as- 
tronomy, —  on  the  occurrence  of  an  eclipse  of  the  sun  in  763 
B.C.,  and  on  the  records  of  the  astronomer  Ptolemy  as  given 
in  his  great  work  the  Almagest. 

There  are  no  more  interesting  portions  of  the  world's 
early  history  than  that  connected  with  the  lands  contiguous 
to  the  Persian  Gulf.  In  Egypt,  the  gigantic  pyramids,  the 
construction  of  which  is  justly  regarded  as  one  of  the  seven 
wonders  of  the  world,  and  the  ancient  temples  point  to  a 
civilization  that  was  in  a  high  state  of  development  five 
thousand  years  ago.  Across  the  narrow  Red  Sea,  more  in- 
teresting even  than  the  land  of  the  Pharaohs,  is  the  country 
of  the  Holy  Scriptures  where  lived  Adam  and  Noah,  the 
great  King  David,  the  holy  prophets  and  the  Lord  Jesus 
Christ.  Up  to  the  middle  of  the  nineteenth  century  little 
was  known  of  ancient  Babylonia  and  Assyria  other  than  the 
account  given  in  the  Bible.  The  country  which  is  nearly 
enclosed  by  the  two  great  rivers  the  Tigris  and  Euphrates 
from  Bagdad  to  the  Persian  Gulf  is  bounded  on  the  north 
by  Mesopotamia,  on  the  east  by  the  plain  of  Elam,  on  the 
south  by  the  Persian  Gulf  and  on  the  west  by  the  Arabian 
Desert.  To  the  north,  in  Mesopotamia,  the  country  is  more 
or  less  mountainous,  while  to  the  south  the  terrain  is  flat 
and  marshy.  In  ancient  times  the  plain  was  covered  with  a 
complicated  network  of  canals.  The  remarkable  fertility 
of  the  soil  brought  such  great  prosperity  to  these  peoples 
that  they  aspired  to  be  the  masters  of  the  world.  According 
to  Herodotus  (i,  193),  wheat  commonly  returned  two 
hundred-fold  to  the  sower,  while  Pliny  (N.  H.  xviii,  17) 
states  that  it  was  cut  twice  each  year.  The  native  historian 
Berossus  remarked  that  wheat,  barley,  palms,  apples  and 
many  kinds  of  shelled  fruit  grew  wild.  The  country  was 
studded  thickly  with  towns.  But  alas,  —  the  neglect  of  the 
canals  has  changed  the  face  of  the  land  so  completely  that 
the  fertility  that  was  once  the  wonder  of  the  ancient  world 
has  departed,  and  in  its  place  there  is  nothing  but  a  barren 
and  desolate  waste,  part  of  the  year  the  land  being  a  series 
of  swamps  and  marshes. 


EARLY    HISTORICAL    ECLIPSES  11 

The  ancient  city  of  Babylon  was  on  the  east  bank  of  the 
Euphrates  about  seventy  miles  south  of  Bagdad.  The  plain 
of  Babylonia  was  called  Edin,  the  "  Eden  "  of  Genesis  ii. 
The  chief  city  of  Assyria,  Assud,  was  on  the  west  side  of  the 
Tigris,  while  the  three  other  great  cities  of  Nineveh,  Calah, 
and  Arbela  were  on  the  eastern  bank  of  the  Tigris. 

The  excavations  at  Nineveh  by  Botta  and  Layard  opened 
up  a  new  world.  Layard  after  indescribable  difficulties  laid 
bare  the  palace  of  Sennacherib  (705—680  B.C.)  and  that  of 
Assurbanipal  (668-626  B.C.),  and  in  the  palace  of  the  latter 
was  found  a  great  library  of  tablets  which  finally  numbered 
no  less  than  thirty  thousand.  These  objects  when  not  too 
massive  were  transported  to  the  Museum  of  the  Louvre 
or  to  the  British  Museum.  The  success  of  these  discoveries 
stirred  scholars  throughout  the  world,  and  the  different 
nations  vied  with  each  other  in  the  prosecution  of  the  inter- 
esting search.  The  British  and  French  continued  their  in- 
vestigations, and  in  1886  German  archaeologists  began  their 
work.  Since  1888  expeditions  have  been  organized  by  the 
University  of  Pennsylvania  and  since  1903  by  the  University 
of  Chicago. 

The  language  in  which  the  ancient  history  is  preserved 
is  the  so-called  Babylonian-Assyrian  wedge-writing,  the 
cuneiform  language  of  signs.  No  such  thing  as  an  alphabet 
was  known.  As  the  $  sign  means  much  to  the  American, 
while  to  the  resident  of  the  British  Isles  the  synonym  of 
money  is  conveyed  by  a  different  symbol,  £,  so  these  primi- 
tive people  expressed  themselves  in  their  tablets  entirely  by 
signs.  A  wedge  was  used  to  make  an  impression  in  clay,  and 
the  different  combinations  served  to  express  a  variety  of 
ideas.  The  decipherment  of  these  tablets  was  a  difficult 
task.  The  most  important  progress  was  made  through  the 
work  of  an  Englishman,  Henry  C.  Rawlinson,  who  observed 
at  Behistun  a  rock  stretching  up  almost  1700  feet  above  the 
plain,  and  at  a  point  about  350  feet  above  its  base  there  was 
a  large  space  carefully  smoothed  off.  On  this  was  found 
a  mass  of  inscriptions  distributed  in  columns  of  various 
lengths.  After  long  years  of  work  he  succeeded  in  trans- 
cribing the  whole  of  the  record.  Careful  study  and  long 


12  ECLIPSES    OF    THE    SUN 

and  arduous  toil  revealed  the  fact  that  the  writing  was  in 
three  different  languages,  old  Persian,  Elamite  and  the 
Babylonian.  The  decipherment  was  excessively  difficult  due 
to  the  enormous  number  of  signs  employed.  For  simple 
names  there  were  about  600  independent  and  distinct  signs 
made  by  combinations  of  wedges  numbering  anywhere  from 
two  to  thirty.  But  two  to  six  of  these  signs  might  be  com- 
pounded to  express  more  complicated  ideas,  with  the  result 
that  there  are  something  like  20,000  different  signs  known 
to  students  of  Assyriology.  The  number  of  inscriptions  so 
far  found  number  about  one  hundred  and  fifty  thousand. 

The  earliest  mention  of  Babylon  appears  to  be  about 
3800  B.C.  The  first  great  king  was  Sargon  of  Akkad  who 
lived  about  2750  B.C.  Even  greater  was  Khammurabi  the 
Amorite  who  flourished  about  2100  B.C.  Concerning  this 
latter  monarch,  who  lived  over  four  thousand  years  ago, 
we  have  positive  information  in  a  group  of  fifty-five  of  his 
letters  edited  by  L.  W.  King,  and  in  a  monument  giving 
a  code  of  splendid  laws.  From  his  letters  we  learn  that  the 
Babylonians  kept  their  record  of  time  entirely  by  the  moon, 
a  new  month  beginning  with  each  new  moon.  Twelve  lunar 
months  made  up  the  year.  Since  the  synodic  month  is  of 
twenty-nine  and  a  half  days  in  length,  it  became  necessary 
to  add  in  the  reckoning  of  the  year  a  thirteenth  month  from 
time  to  time  in  order  that  the  calendar  should  not  get  astray. 

This  method  of  measuring  the  length  of  the  year,  which  is 
both  inaccurate  and  inconvenient,  was  followed  by  the 
Hebrews  and  the  Persians.  Even  in  the  enlightened  age  of 
the  twentieth  century  the  Jewish  and  Mohammedan  cal- 
endars are  still  based  on  the  old  custom.  An  intercalary 
month  being  inserted  in  the  reign  of  Khammurabi,  he  ac- 
cordingly sent  out  a  circular  letter  to  his  governors  advising 
them  that  the  delinquent  tax  gatherers  should  not  take  ad- 
vantage of  the  change  in  the  calendar  and  that  the  taxes 
must  be  paid  without  delay.  Schools  flourished  even  at 
this  remote  period,  and  tablets  have  been  found  which  evi- 
dently were  exercises  of  the  children  as  they  struggled  to 
learn  the  language  of  signs.  In  fact  a  Babylonian  proverb 


BRIGHTER  PORTIONS  OF  THE  CORONA  SHOWING  SAME  REGION  AS  PAGE  8 
Photograph  by  Marriott  with  63-foot  tower  telescope. 


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EARLY    HISTORICAL    ECLIPSES  13 

has  been  unearthed  which  reads,  "  He  who  shall  excel  in 
tablet-writing  shall  shine  like  the  sun." 

The  kingdom  of  Assyria  begins  about  3000  B.C.  Assyria 
was  a  warlike  nation,  and  lived  by  the  sword.  In  the  eighth 
century  B.C.,  their  conquests  had  been  pushed  as  far  west- 
ward as  Damascus  and  to  the  Hebrew  city  of  Samaria. 
While  besieging  this  latter  city,  the  king  was  deposed  and  a 
usurper  called  Sargon  took  the  throne.  Under  him  and  his 
son  Sennacherib,  Assyria  was  at  the  zenith  of  its  military 
power.  Campaigns  were  carried  out  in  Ionia  and  Greece 
and  also  southward  as  far  as  Egypt,  which  became  tributary 
to  Assyria.  In  Nineveh,  Sennacherib  constructed  the  largest 
and  most  magnificent  palace  the  world  had  seen  up  to  this 
time.  In  689  B.C.  he  conquered  the  city  of  Babylon,  and  he 
caused  this  holy  city  to  be  completely  destroyed  and  all 
of  its  buildings  razed  to  the  ground.  After  many  years 
Babylon  under  Nabopolassar,  revolted  from  the  Assyrian 
yoke  and  under  his  son  Nebuchadrezzar  —  the  Nebuchad- 
nezzar of  the  Bible  —  construction  on  a  great  scale  was 
carried  out,  and  Babylon  was  rebuilt  and  became  the  wonder 
city  of  the  ancient  world.  According  to  Herodotus,  the 
walls  were  no  less  than  56  miles  in  circumference,  the  walls 
being  335  feet  high  and  85  feet  wide.  Each  city  had  its 
great  temple  and  its  own  god,  the  god  of  Babylon  being 
Bel-Merodach.  In  the  temple  stood  an  enormous  image  of 
the  god  forty  feet  high,  together  with  a  table,  a  mercy  seat 
and  an  altar,  all  constructed  of  gold.  In  this  reign  (604- 
561  B.C.),  the  rebuilt  Babylon  was  at  the  height  of  her 
glory  and  the  summit  of  Chaldean  civilization  had  been 
reached. 

This  glimpse  of  the  history  of  these  early  peoples  of  such 
great  interest  to  all  Christian  countries  is  for  the  purpose  of 
giving  a  setting  for  ascertaining  what  they  had  accomplished 
in  art,  literature  and  science  —  particularly  in  the  science  of 
astronomy.  Assyria  differed  much  from  Babylonia.  The 
former  state  was  an  armed  camp  and  held  sway  by  the 
sword,  probably  even  in  those  early  times  believing  that 
might  is  right.  On  the  other  hand,  the  Babylonians  were 
peace-loving  people,  a  land  of  merchants  and  farmers,  and 


14  ECLIPSES    OF    THE    SUN 

withal  deeply  religious.  Consequently,  Assyria  had  little 
culture  of  its  own  and  the  little  it  did  possess  was  borrowed 
or  acquired  from.  Babylonia. 

One  of  the  grandest  epics  of  ancient  Babylonia  was  the 
description  of  Creation  which  is  mentioned  here  on  account 
of  its  astronomical  significance  and  also  that  it  may  be  com- 
pared with  the  Biblical  description.  In  the  first  book  an 
account  is  given  of  the  creation  of  the  world  out  of  the 
primeval  deep  and  the  birth  of  the  gods  of  light.  Then 
comes  the  story  of  the  struggle  between  the  gods  of  light 
and  the  powers  of  darkness,  and  the  final  victory  of  Mero- 
dach, who  clove  the  dragon  of  chaos,  Tiamat,  asunder, 
forming  the  heaven  out  of  one  half  of  her  body  and  the 
earth  out  of  the  other.  Merodach  next  arranged  the  stars 
in  order,  along  with  the  sun  and  moon,  and  gave  them  laws 
which  they  were  never  to  transgress.  After  this  the  plants 
and  animals  were  created,  and  finally  man.  Merodach  here 
takes  the  place  of  Ea,  who  appears  as  the  creator  in  the  older 
legends,  and  is  said  to  have  fashioned  man  out  of  clay. 

Thus  from  very  earliest  times,  the  religion  of  the  Baby- 
lonians was  inspired  by  the  belief  that  the  heavenly  bodies 
were  subject  to  law  and  order  and  that  they  ruled  the  des- 
tinies of  man.  It  was  evident  even  to  the  simple  mind  of  the 
primitive  man  that  the  sun  was  the  cause  of  all  life  on  the 
earth,  and  it  was  a  natural  conclusion  that  the  ever-changing 
aspect  of  the  sun,  moon,  planets  and  stars  should  be  con- 
nected with  the  constant  mutations  in  the  lives  of  the  in- 
dividuals of  the  race.  Hence  their  gods  and  goddesses  were 
identified  with  the  heavenly  bodies.  The  priests  became 
astrologers.  To  be  able  to  read  the  signs  of  the  heavens 
seemed  to  be  synonymous  with  understanding  the  happen- 
ings on  earth,  and  consequently  if  this  knowledge  were  to 
be  utilized  to  predict  what  would  take  place  in  the  future 
it  was  necessary  to  follow  with  the  minutest  care  the  vari- 
ous motions  of  the  sun,  moon,  planets  and  stars  —  and 
hence  the  priests  became  astronomers.  These  religious  be- 
liefs caused  a  systematic  study  of  the  heavenly  bodies;  and 
as  a  result  astronomy,  the  first  and  parent  of  all  of  the 
Sciences,  had  its  birth. 


EARLY    HISTORICAL    ECLIPSES  15 

The  movements  of  the  sun  and  moon  were  symbols  of  law 
and  order,  and  accordingly  they  were  worshipped  as  gods. 
The  motions  of  the  planets,  though  more  difficult  to  under- 
stand, seemed  also  the  subject  of  order  and  not  of  caprice, 
and  so  likewise  they  were  worshipped.  Jupiter  was  the 
abode  of  the  Babylonian  god  Mardok,  and  Venus  was  the 
home  of  the  goddess  of  love,  Ishtar.  Saturn  was  identified 
with  Ninib,  Mercury  with  Nebo,  and  Mars  with  Nergal. 
In  order  to  understand  the  manner  in  which  the  gods  of  the 
sun,  moon  and  planets  influenced  the  lives  of  men,  it  was 
necessary  for  the  Babylonian  priests  to  observe  the  position 
of  these  bodies,  not  only  with  respect  to  each  other,  but  also 
with  respect  to  the  more  prominent  and  easily  recognizable 
fixed  stars.  In  the  case  of  the  moon  it  was  very  necessary 
to  note  the  time  at  which  the  new  moon  became  visible  as 
a  crescent,  its  position  in  the  heavens,  and  the  angle  made 
with  the  horizon  by  the  line  through  the  horns.  (Even  at 
the  present  supposedly  enlightened  age  of  the  twentieth 
century  A.D.,  we  still  hear  of  the  "  wet "  moon,  and  the 
"  dry  "  moon.)  The  changes  of  the  moon  were  followed 
with  meticulous  care  since  each  change  afforded  the  op- 
portunity of  interpreting  some  phase  of  human  activity. 
And  through  forty  centuries  of  the  world's  history  some  of 
these  superstitions  have  survived  in  the  popular  belief  that 
a  change  in  the  moon  means  a  change  in  the  weather,  or 
that  certain  crops  should  be  planted  "  in  the  dark  of  the 
moon."  These  observations  of  the  heavenly  bodies  being 
thus  gradually  accumulated  must  have  reached  a  mass  of 
considerable  proportion.  For  his  interpretation  of  these  ob- 
servations the  astrologer  rested  on  written  records  or  on 
the  recollection  of  what  had  happened  under  similar  circum- 
stances in  the  past,  but  sometimes  merely  on  the  vagueness 
of  association  of  ideas. 

The  motion  of  the  sun  and  moon  among  the  stars  near  the 
ecliptic  being  carefully  observed,  it  was  but  natural  that 
the  twelve  full  moons  of  the  year  should  suggest  the  divi- 
sion of  the  circle  of  the  year  into  twelve  parts.  We  thus 
owe  to  the  Babylonians  the  constitution  and  nomenclature 
of  the  twelve  signs  of  the  zodiac.  Ages  before  AssurbanipaPs 


16  ECLIPSES    OF    THE    SUN 

great  library  of  tablets  was  constructed,  the  eighth  month 
was  known  as  "  the  month  of  the  star  of  the  Scorpion/'  the 
tenth  as  "  the  star  of  the  Goat/'  while  the  twelfth  was  the 
month  of  "  the  star  of  the  Fish  of  Ea."  The  convenience  of 
the  duodecimal  system  was  thus  early  recognized.  Each 
day  was  divided  into  twelve  "  double  hours. "  The  ner  of 
600,  the  sar  of  3600  was  formed  from  the  soss,  or  unit  of 
60.  The  circle  was  divided  into  360°,  or  six  times  sixty, 
with  further  divisions  by  60  into  minutes  and  seconds  of 
arc,  the  hours  being  likewise  divided  into  minutes  and 
seconds  of  time.  According  to  Miss  A.  M.  Clerke,  "  In  the 
Chaldean  signs  of  the  zodiac,  fragments  of  several  distinct 
strata  of  thought  appear  to  be  subdivided.  From  one  point 
of  view  they  shadow  out  the  great  epic  of  the  destinies  of  the 
human  race;  again  the  universal  solar  myth  claims  a  share 
in  them;  hoary  traditions  were  brought  into  ex  post  facto 
connection  with  them;  or  they  served  to  commemorate  sim- 
ple meteorological  and  astronomical  facts. "  Astronomy 
was  thus  of  very  old  standing  in  Babylonia.  The  principal 
astronomical  work,  called  the  Illumination  of  Bel  was 
compiled  for  the  library  of  Sargon  of  Akkad ;  it  was  inscribed 
on  70  tablets,  and  apparently  went  through  numerous  edi- 
tions, one  of  the  tablets  being  in  the  British  Museum.  It 
treats,  among  other  things,  on  observations  of  comets,  the 
pole  star,  the  conjunction  of  sun  and  moon,  and  the  motions 
of  Venus  and  Mars. 

However,  since  the  study  of  orderly  motions  of  the 
heavenly  bodies  was  primarily  for  the  purpose  of  forecasting 
human  events,  it  is  not  surprising  to  learn  from  modern 
researches  that  the  early  astronomical  knowledge  was  very 
crude,  with  very  little  perfection  in  the  observations.  It 
is  true  that  as  early  as  the  days  of  Khammurabi  (2 100  B.C.) 
there  were  combinations  of  prominent  groups  of  stars  into 
outlines  of  animals  and  figures,  yet  there  is  no  evidence  that 
prior  to  700  B.C.  more  than  a  small  number  of  the  constella- 
tions of  the  zodiac  had  found  their  place  in  the  sky. 

The  Babylonians  had  tables  of  squares  and  cubes  calcu- 
lated from  i  to  60,  they  measured  time  by  the  sun-dial  and 
by  water  clocks,  were  familiar  with  lever  and  pulley,  and 


EARLY    HISTORICAL   ECLIPSES  17 

even  possessed  a  lens  turned  on  a  lathe,  but  which  was  cer- 
tainly not  used  as  a  magnifying  glass.  Such  proficiency  in 
this  early  people  would  lead  one  to  expect  that  though  their 
observations  may  have  been  crude  they  had  nevertheless 
perfected  systems  of  moon  calculation  and  planetary  tables 
of  a  high  order  of  excellence.  The  discovery  of  the  Saros  of 
223  lunations,  so  useful  and  important  in  the  predicting  of 
eclipses,  has  been  attributed  to  the  Chaldeans.  They  seem 
also  to  have  been  aware  that  Venus  returns  in  almost  ex- 
actly eight  years  to  the  same  point  in  the  sky  and  to  have 
established  similar  relations  of  46  years  for  Mercury,  59 
for  Saturn,  79  for  Mars  and  83  for  Jupiter.  They  were 
thus  able  to  fix  in  advance  the  positions  of  the  heavenly 
bodies,  of  such  great  importance  in  astrological  lore.  In 
fact  it  is  generally  thought  that  Hipparchus,  the  first  great 
astronomer  of  the  world,  acquired  much  of  his  information 
from  the  Chaldeans,  though  unquestionably  he  verified 
all  of  their  findings.  The  Babylonian  sage  Berossus  founded 
a  school  about  640  B.C.  in  the  island  of  Cos,  and  it  is  not 
impossible  that  Thales  of  Miletus  was  one  of  his  pupils. 

In  spite  of  the  unbounded  admiration  which  one  must 
feel  for  the  development  of  astronomy  under  the  Chaldeans 
it  seems  much  more  plausible  to  believe  that  the  greatest 
work  of  the  Chaldeans  was  accomplished  in  the  later  years 
of  their  history,  this  taking  place  even  after  the  fall  of 
Babylon  in  539  B.C.,  and  in  fact  after  the  Greeks  had  in- 
vaded the  valley  of  the  Euphrates.  This  matter,  and  partic- 
ularly their  acquaintance  with  the  Saros,  will  be  further 
discussed  in  dealing  with  the  famous  eclipse  of  Thales. 

The  study  of  astrology  thus  securing  a  firm  foundation 
under  the  Babylonians,  it  spread  from  them,  directly  or  in- 
directly, to  all  quarters  of  the  globe.  It  came  to  Greece 
about  the  middle  of  the  fourth  century  B.C.,  and  reached 
Rome  before  the  opening  of  the  Christian  era.  In  India 
and  China  both  astronomy  and  astrology  were  acquired  from 
the  Greeks,  and  are  largely  reflections  of  Greek  theories 
and  speculations.  By  the  introduction  of  Greek  culture 
into  Egypt,  astronomy  and  astrology  were  both  cultivated 
in  the  land  of  the  ancient  Pharaohs  during  the  Greek  and 


18  ECLIPSES    OF    THE    SUN 

Roman  periods.  The  Arabs  developed  astrology  and  also 
astronomy,  many  of  the  names  of  the  brighter  stars  being 
Arabic.  In  Europe  as  late  as  the  i4th  and  isth  centuries, 
astrologers  had  important  positions  at  the  royal  courts 
and  were  consulted  on  all  matters  of  great  moment  to  the 
nation.  With  the  revival  of  learning,  and  particularly  after 
the  development  of  the  Copernican  system  which  showed 
that  the  earth  was  but  one  of  the  planets,  astrology  became 
more  and  more  pushed  into  the  background,  though  as 
late  as  the  iyth  century  horoscopes  were  cast  by  the  great 
astronomer  Kepler.  Even  in  the  United  States  of  America 
and  in  England  there  are  still  many  thousands  of  people 
who  believe  in  the  flat  earth,  and  who  have  great  confidence 
in  the  efficacy  of  horoscopes  to  correctly  forecast  the  future. 
Traces  of  the  Babylonian  astrology  are  still  found  in  our 
every-day  language,  for  we  still  have  such  phrases  as  "  I 
thank  my  stars,"  he  was  "  born  under  a  lucky  star,"  or  an 
"  ill-starred  undertaking." 

As  already  stated,  the  Babylonians  had  a  calendar  of 
twelve  lunar  months,  and  a  seven-day  week,  the  seventh 
day  being  a  day  of  rest.  The  early  Babylonians  reckoned 
events  from  some  great  catastrophe,  such  as  in  our  day, 
Chicago  and  its  great  fire,  but  later  in  their  history  they 
counted  time  by  the  years  of  the  reigning  king.  There  were 
several  early  dynasties,  but  the  succession  of  rulers  is  fairly 
certain.  In  Assyria,  on  the  contrary,  a  plan  unique  among 
the  early  peoples  was  followed  in  naming  the  year  after 
officers,  called  eponyms,  whose  term  extended  for  one  year 
only.  This  "  Eponym  Canon,"  as  it  is  called,  began  with 
the  year  911  B.C.  In  the  Almagest  of  Ptolemy  there  is  a 
list  of  Babylonian,  Assyrian  and  Persian  kings  who  ruled 
in  Babylon  together  with  the  years  each  of  them  reigned  be- 
ginning with  the  accession  of  Nabonassar  in  747  B.C.,  to 
the  conquest  of  Babylon  in  33 1  B.C.  by  Alexander  the  Great. 
This  Ptolemaic  Canon  is  confirmed  by  other  Babylonian 
chronicles  and  also  by  the  Assyrian  Eponym  Lists. 

These  Assyrian  tablets  record  three  eclipses  of  the  sun. 
The  first,  interpreted  by  Rawlinson  in  1867,  is  referred  to 
as  follows:  "  In  the  Eponymy  of  Burgasole,  Governor  of 


EARLY    HISTORICAL    ECLIPSES  19 

Gozan,  a  revolt  in  Assur  took  place  in  the  month  Sivar, 
and  the  sun  was  eclipsed."  To  call  attention  to  the  im- 
portance of  the  event  the  ancient  scribe  drew  a  line  across 
the  tablet.  This  eclipse  has  been  carefully  investigated  by 
a  number  of  astronomers,  by  Ginzel,  Airy,  Hind  and  others, 
and  the  general  conclusion  is  that  Nineveh,  where  lived  the 
Assyrian  scribe,  was  just  outside  the  path  of  totality,  and 
that  the  greatest  obscuration  was  about  9:47  in  the  morning 
of  June  15,  763  B.C.  The  second  eclipse  mentioned  in  the 
Assyrian  records  took  place  in  the  reign  of  Esar-haddon, 
the  son  and  successor  of  Sennacherib.  This  eclipse  was 
partial  in  Assyria,  but  annular  farther  to  the  east,  and  the 
date  was  May  27,  669  B..C.  The  record  of  the  third  is  a 
little  uncertain,  but  it  is  probably  the  eclipse  of  June  27, 
66 1  B.C.,  during  the  reign  of  Assurbanipal. 

These  are  not  the  only  eclipses  observed  by  the  Chaldeans, 
for  in  the  Almagest  of  Ptolemy  is  a  record  of  three  eclipses 
of  the  moon  observed  in  Babylon.  The  first  of  the  three 
was  a  total  eclipse  on  March  19,  721  B.C.,  while  the  other 
eclipses,  partial  only,  were  observed  the  following  year, 
on  March  8,  and  September  i.  As  has  been  stated  above, 
Ptolemy  gave  a  list  of  the  kings  who  reigned  in  Babylonia. 
These  eclipses,  of  sun  and  moon,  fix  the  dates  of  Eastern 
chronology  with  great  exactness,  the  earliest  date  in  the 
world's  history  to  be  thus  accurately  determined  being  the 
year  911  B.C.  in  Assyrian  chronology.  Earlier  dates  are 
known  with  increasing  uncertainty.  The  various  estimates 
of  historians  for  the  beginning  of  the  first  Egyptian  dynasty 
differ  as  much  as  two  thousand  years! 

As  a  result  of  the  Babylonian  eclipses,  it  has  been  neces- 
sary to  alter  the  chronology  of  the  Bible  by  lowering  the 
dates  to  the  extent  of  twenty -four  years.  This  will  be  re- 
ferred to  later  in  connection  with  passages  from  the  Holy 
Scriptures  in  Amos  and  Isaiah. 

EGYPTIAN  ASTRONOMY 

In  the  neighboring  country  of  Egypt  excellent  opportunity 
must  have  been  afforded  for  a  study  of  astronomy  on  ac- 


20  ECLIPSES    OF    THE    SUN 

count  of  the  clear  skies  of  the  East  and  from  the  intercourse 
which  must  certainly  have  taken  place  between  the  Egyp- 
tians and  the  Chaldeans  of  Asia  Minor.  Egyptian  records 
accordingly  have  been  carefully  scrutinized  to  find  what 
traces  there  are  of  early  astronomical  knowledge  in  the  land 
of  the  Nile.  And  since  a  study  of  eclipses  must  concern 
itself  with  the  beginnings  of  astronomy,  a  brief  glance  will 
be  given  here  to  early  history  in  Egypt. 

The  early  Chaldean  monuments  are  probably  of  an  earlier 
date  than  those  of  Egypt,  but  the  former  are  almost  form- 
less piles  of  sundried  brick,  while  the  tombs  and  pyramids 
of  the  early  Egyptian  dynasties  are  many  of  them  in  excel- 
lent preservation.  The  history  of  Egypt  is  generally  di- 
vided into  five  periods:1  (i)  The  Ancient  Empire,  3400- 
2160  B.C.,  comprising  ten  dynasties  of  kings;  (2)  the  Middle 
Empire,  with  Thebes  as  capital,  two  dynasties;  (3)  the 
second  Thebic,  or  New  Empire  (1588-1150  B.C.),  compris- 
ing dynasties  XVII-XX,  separated  from  the  Middle  Em- 
pire by  the  Shepherd  Kings  of  Arabia;  (4)  the  Decadence 
period  of  six  dynasties,  1150-324  B.C.,  which  includes  the 
Persian  conquest  in  525  B.C.;  and  (5)  the  Ptolemaic  and 
Roman  period,  324  B.C.-300  A.D. 

Little  is  known  of  exact  dates  before  the  Conquest  by 
Alexander  although  repeated  attempts  have  been  made  to 
determine  them.  The  list  of  the  succession  of  the  Egyptian 
kings  is  known  with  certainty,  but  the  chronology  is  uncer- 
tain for  the  following  reasons:  (i)  The  lengths  of  the  in- 
dividual reigns  are  not  known  with  precision,  since  the 
record  seldom  reached  to  the  end  of  the  reign,  and  did  not 
allow  for  co-regencies.  (2)  Calculations  on  the  probable 
length  of  a  period  leads  to  no  trustworthy  information.  (3) 
Comparisons  with  other  records,  particularly  with  those  of 
Babylonia  and  Assyria,  give  the  most  valuable  knowledgee 
But  the  dates  before  911  B.C.  are  not  known  accurately  even 
in  Assyrian  chronology,  and  it  is  therefore  not  surprising 
that  the  date  of  the  beginning  of  the  XlXth  dynasty  should 
be  estimated  by  competent  authorities  anywhere  from  1490 
B.C.  to  1315  B.C.  (4)  Of  most  interest  to  us  here  is  the 

1  Hamlin,  Encyclopedia  Americana,  10,  12. 


EARLY    HISTORICAL    ECLIPSES  21 

astronomical  information.  The  Egyptian  number  system 
was  decimal,  each  power  of  10  up  to  100,000  being  repre- 
sented by  a  different  figure,  on  much  the  same  principle  as 
the  Roman  numerals.  The  day  was  divided  into  two  periods, 
each  of  twelve  hours,  the  beginning  of  the  day  being  the  time 
of  sunrise.  The  year  was  divided  into  twelve  months,  each 
of  thirty  days,  or  a  year  of  360  days.  As  early  as  the  Vth 
dynasty,  the  premature  arrival  of  the  seasons  was  noted, 
and  as  a  consequence  five  complementary  days  were  added 
making  a  year  of  365  days.  The  extra  days  were  counted 
either  at  the  beginning  or  at  the  end  of  the  year.  The  year 
was  divided  into  three  seasons,  each  month  into  three  weeks, 
each  of  ten  days.  Since  the  season  of  agricultural  growth 
depended  more  on  the  inundation  by  the  Nile  than  on  the 
motion  of  the  sun,  the  first  of  the  year  was  reckoned  as  the 
beginning  of  the  rise  of  the  waters  of  the  river.  As  this  took 
place  with  fair  regularity,  the  Egyptians  were  thus  furnished 
with  a  useful  starting-point  for  their  annual  counting  of 
time.  It  was  noticed  that  the  brilliant  dog-star,  Sirius,  or 
Sothis,  rose  with  the  sun  about  this  time  (July  19),  and  as 
this  star  is  so  brilliant,  refined  astronomical  measurements 
were  not  necessary  in  order  to  observe  it.  But  the  tropical 
year,  the  year  of  the  seasons,  is  approximately  a  quarter  of 
a  day  more  than  365  days.  (The  tropical  year  is  now  known 
to  be  equal  to  365.2422  mean  solar  days.)  Hence  the 
Egyptian  calendar  got  astray  one  month  every  121  years, 
or  one  year  every  1461  years.  This  period,  during  which 
the  New  Year's  day  of  the  Egyptians  traveled  all  round 
the  calendar,  was  known  to  Greek  and  Roman  writers 
in  the  first  century  B.C.  Would  that  Julius  Caesar  when 
revising  the  Calendar  —  the  Julian  being  the  basis  of 
the  Gregorian,  or  modern  calendar  —  had  followed  the  sim- 
plicity of  the  Egyptian  calendar,  at  least  in  keeping  each 
month  of  a  uniform  number  of  days,  and  would  that  pride 
and  jealously  had  not  robbed  poor  February  of  two  days! 
The  modern  calendar  is  very  awkward  and  inconvenient,  and 
there  have  been  many  attempts  suggested  for  the  purpose  of 
revising  it.  Most  of  the  modern  revisions  proposed  call  for 
an  extra  day  in  the  year,  or  two  extra  days  in  leap  years, 


22  ECLIPSES    OF    THE    SUN 

which  days  are  not  to  be  included  in  the  reckoning  of  any 
week  or  any  month.  Attention  should  be  called  to  the  fact 
that  the  five  extra  days  of  the  Egyptian  year  —  recorded  as 
early  as  the  Vth  dynasty  —  were  regarded  as  so  peculiarly 
unlucky  that  there  is  not  a  single  instance,  among  the  many 
thousands  of  inscriptions  brought  to  light,  of  any  contract 
being  entered  into  on  any  one  of  the  five  complementary 
days,  nor  has  any  event  of  importance  ever  been  recorded 
as  taking  place  on  one  of  these  unlucky  days. 

The  brilliant  skies  of  Egypt  favored  the  development  of 
observations.  It  is  generally  believed  that  the  Egyptians 
observed  the  motions  of  the  heavenly  bodies  for  the  purpose 
of  fixing  the  dates  of  their  religious  festivals.  These  times 
were  probably  noted  by  observing  the  times  at  which  certain 
of  the  brighter  stars  arose  at  dawn  just  before  the  sun.  For 
accomplishing  this  object  no  instruments  were  necessary. 

They  also  determined  the  hours  of  the  night  by  observing 
the  meridian  passage  of  certain  stars.  What  is  thought  to  be 
the  oldest  type  of  observing  instrument  in  the  world,  known 
to  the  Egyptians  as  merkhet,  was  probably  utilized  for  this 
purpose.  It  consisted  of  a  handle  supporting  a  plumb  bob 
and  a  reed  which  served  as  a  sighting  vane.  By  means  of 
this  and  with  the  help  of  the  ancient  Egyptian  horoscopus, 
or  clock,  the  meridian  could  be  laid  out  and  time  determined 
by  observation.  There  have  been  preserved  the  titles  of 
several  temple  books,  which  books  apparently  recorded  the 
movements  of  the  sun,  moon  and  stars,  but  unfortunately, 
not  a  single  one  of  these  records  themselves  have  survived. 
The  Egyptians  were  of  an  intensely  practical  turn  of  mind, 
and  they  had  little  desire  to  acquire  knowledge  for  the  sake 
of  the  knowledge  itself.  If  the  information  could  be  utilized 
for  any  practical  use  it  was  carefully  cherished  and  pre- 
served, or  if  it  could  assist  in  their  religious  speculations 
it  was  then  regarded  of  great  value.  If  we  are  to  judge  of 
the  accomplishments  of  the  Egyptians  by  the  written  docu- 
ments alone,  we  must  assume  that  they  knew  little  and 
cared  less  for  any  branch  of  science.  But  then,  there  are  the 
pyramids,  and  the  great  temples  such  as  Karnak!  Surely 
the  architects  and  engineers  who  constructed  such  colossal 


EARLY    HISTORICAL    ECLIPSES  23 

monuments  must  have  possessed  knowledge  of  surveying 
vastly  greater  than  is  shown  by  any  written  record.  The 
construction  of  the  Great  Pyramid  apparently  took  place  less 
than  a  century  and  a  half  after  the  elevation  of  the  earliest 
piece  of  stone  masonry  known.  Since  these  pyramids  could 
have  been  erected  only  by  means  of  great  mechanical  power, 
we  are  forced  to  the  conclusion  that  the  progress  in  the  con- 
trol of  such  power  was  greater  at  the  thirtieth  century  B.C. 
than  at  any  other  age  of  the  world's  history  except  the 
present.  The  pyramid  was  placed  four  square  to  the  points 
of  the  compass,  and  leading  to  the  sepulchral  tomb  there  was 
a  passage  at  an  angle  of  26°  to  the  horizon,  which  has  been 
thought  furnished  a  corridor  for  observing  the  star  which 
was  then  the  pole  star,  a  Draconis.  With  this  assumption, 
an  estimation  has  been  made  of  the  possible  age  of  the  pyra- 
mids. According  to  C.  Piazzi  Smyth,  the  passage  in  the 
pyramid  facing  the  south  was  used  for  the  observation  of 
the  meridian  passage  of  the  stars  and  planets,  and  it  is  even 
possible  that  the  moon  and  sun  were  similarly  observed. 
Proctor  thinks  that  if  the  Egyptians  had  utilized  an  opaque 
screen  with  a  small  round  hole  in  it  at  the  southern  end  of 
this  gallery  they  could  even  have  observed  sun-spots! 

It  is  plausible  to  believe  that  the  Great  Pyramid  was  pos- 
sibly designed  to  serve  as  an  astronomical  observatory,  but 
the  truth  is  that  not  a  single  competent  Egyptologist  sup- 
ports this  view.  As  Breasted  has  pointed  out,  the  careful 
orientation  of  the  pyramid  with  its  passage  directed  towards 
the  north  star  is  in  keeping  with  the  popular  belief  that  the 
king  was  transmuted  into  a  star,  —  hence  the  shaft  directed 
towards  those  stars  in  the  sky  which  never  set. 

According  to  E.  W.  Maunder,  the  symbol  of  the  sun-god, 
so  frequently  found  on  Egyptian  monuments,  is  sufficient 
evidence  to  prove  that  total  eclipses  of  the  sun  were  ob- 
served in  Egypt.  The  general  design  of  this  symbol  is  a 
circle  with  striated  wings  to  right  and  to  left,  and  with 
a  lesser  extension  in  the  downward  direction,  but  with 
nothing  above  the  circle.  As  pointed  out  by  Maunder, 
the  extensions  to  right  and  left  call  to  mind  and  re- 
semble the  form  of  the  solar  corona  when  the  eclipse,  oc- 


24  ECLIPSES    OF    THE    SUN 

curring  at  sun-spot  minimum,  has  the  equatorial  wings. 
This  explanation  is  indeed  plausible,  and  it  is  not  impossible 
that  it  represents  the  truth.  To  use  the  coronal  form  for  the 
symbol  of  the  sun  would  have  necessitated  that  the  corona 
had  been  frequently  observed  by  the  Egyptians,  and  at  more 
than  one  age.  Such  symbols  could  be  adopted  by  a  people 
only  after  slow  awakening  and  continued  observation.  Con- 
sequently, if  eclipses  had  been  so  frequently  observed  it 
would  be  rational  to  expect  that  records  of  some  of  them 
would  have  been  preserved.  Considering  the  frequency 
with  which  eclipses  were  observed  in  Babylon,  it  is  surprising 
in  the  highest  degree  to  find  not  a  single  reference  any- 
where in  Egyptian  antiquity  to  an  eclipse,  either  of  the  sun 
or  of  the  moon.  We  are  therefore  forced  to  the  conclusion 
that  the  Egyptians  had  little  part  in  the  development  of  the 
science  of  astronomy. 

The  New  Empire  beginning  with  the  XVIIIth  dynasty 
marks  the  golden  period  of  Egyptian  life.  This  was  the  age 
of  conquest  with  an  empire  stretching  from  the  Euphrates 
to  the  fourth  cataract  of  the  Nile.  Egypt  had  become  a 
great  military  empire  whose  world  power  lasted  from  the 
early  sixteenth  to  the  twelfth  century  B.C.,  or  over  four  hun- 
dred years.  The  capital  was  the  once  vast  city  of  Thebes 
constructed  on  both  sides  of  the  river  Nile,  the  first  great 
monumental  city  built  by  man.  On  the  east  side  of  the  river 
is  the  temple  of  Karnak  which  is  nearly  a  quarter  of  a  mile 
long  and  was  nearly  two  thousand  years  in  the  course  of 
construction.  The  obelisk  of  Queen  Hatshepsut,  the  first 
great  woman  queen  of  history,  is  still  standing.  This  is 
about  one  hundred  feet  high  and  weighs  350  tons.  The 
first  of  the  world's  great  generals  was  Thutmose  III,  who 
ruled  for  fifty  years  beginning  about  1500  B.C.,  and  on  the 
walls  at  Karnak  are  inscribed  the  stories  of  his  great  ex- 
ploits. 

The  recent  discovery  of  the  tomb  of  Tut-ankh-Amen  has 
brought  into  sudden  prominence  that  portion  of  Egyptian 
history  immediately  following  the  reign  of  Thutmose  III. 
While  little  light  so  far  has  been  thrown  on  the  actual  his- 
torical facts  of  the  lives  of  Tut-ankh-Amen  and  his  im- 


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EARLY    HISTORICAL    ECLIPSES  25 

mediate  predecessors,  the  incomparable  beauty  of  the 
objects  of  art  found  within  the  tomb  bids  fair  to  revolu- 
tionize the  history  of  Egyptian  art  and  literature.  It  is  to 
the  influence  of  the  religious  upheaval  brought  about  by 
Khu-n-aten,  the  successor  of  Thutmose  IV,  and  reputed 
father-in-law  of  Tut-ankh-Amen,  that  the  revolution  in 
artistic  and  literary  expression  is  due.  This  monarch, 
known  as  the  "  heretic  king/'  physically  a  weakling,  men- 
tally a  poet  and  dreamer,  revolted  from  the  polytheistic 
religion  of  his  forefathers  and  founded  a  new  faith  which 
deified  the  sun  as  the  giver  of  all  light  and  life.  This 
worship,  with  Aten  the  sun  as  its  visible  symbol,  had  many 
points  in  common  with  Christianity  and  has  been  regarded 
as  the  most  beautiful  of  the  ancient  religions.  In  his  en- 
thusiasm for  the  new  faith,  however,  Khu-n-aten  allowed 
the  material  needs  of  his  empire  to  go  unheeded  to  such 
an  extent  that  at  his  death  the  confusion  and  distress 
throughout  Egypt  was  so  great  that  his  successor  was  forced 
to  give  way  to  the  strong  political  power  of  the  priests  of 
the  old  religion,  and  Amen  once  more  became  the  chief 
Egyptian  god.  It  was  in  the  shadowy  period  between  Khu- 
n-aten  and  Rameses  I  that  Tut-ankh-Amen's  brief  reign 
occurred. 

The  oldest  astronomical  instrument  in  the  world  has  re- 
cently been  discovered  by  Breasted.  The  interest  in  this 
remarkable  discovery  is  all  the  greater  for  the  reason  that 
it  is  believed  that  the  instrument  was  actually  made  by  no 
less  a  personage  than  King  Tut-ankh-Amen  himself.  This 
object,  a  merkhet,  consists  of  a  rectangular  strip  of  ebony 
wood  10  x  i  x  Jr  inches.  Along  each  edge,  extending  from 
end  to  end,  is  an  inscription  which  states  that  the  instrument 
was  made  by  King  Tut-ankh-Amen,  "with  his  two  hands," 
as  a  restoration  of  a  similar  monument  stolen  from  the  tomb 
of  his  ancestor,  Thutmose  IV.  Indeed,  the  tomb  of  the 
latter  contains  a  remark  in  ink  on  the  wall  stating  that  his 
tomb  had  been  restored  (of  course  after  robbery)  by  Harm- 
hab,  who  was  practically  the  successor  of  Tut-ankh-Aijien. 

Apparently  the  ancient  Egyptians  were  quite  accustomed 


26  ECLIPSES    OF    THE    SUN 

to  despoiling  the  tombs  of  their  ancestors;  that  of  Thutmose 
IV  had  already  been  robbed  during  the  reign  of  Tut-ankh- 
Amen  while  that  of  the  latter  king  was  soon  to  follow  a 
similar  fate. 

The  XlXth  dynasty  began  with  Rameses  I,  whose  reign, 
according  to  Breasted,  commenced  about  1315  B.C.  This 
king  reigned  only  two  years,  but  he  had  planned  and  had 
started  construction  on  the  great  colonnaded  hall  at  Karnak, 
the  greatest  of  such  edifices  ever  constructed  by  man.  It  is 
338  feet  long  and  170  feet  wide.  There  are  one  hundred  and 
thirty-six  columns  in  sixteen  rows,  those  in  the  nave  being 
higher  than  the  rest.  Rameses  shared  his  throne  with  his 
son  Seti  I  for  one  year,  and  Seti  continued  the  gigantic 
plans  of  his  father.  In  addition  he  built  a  splendid  temple 
at  Abydos,  and  a  magnificent  tomb  in  the  Valley  of  the 
Tombs  of  the  Kings.  This  galleried  tomb  is  the  very  finest 
of  its  kind.  The  decorations  are  wonderfully  charming,  full 
of  life  and  color  though  perhaps  lacking  some  of  the  strength 
and  character  of  earlier  art.  The  outlines  are  forceful,  the 
postures  natural  and  the  coloring  beautiful.  Egyptian  art 
had  reached  the  pinnacle.  The  Metropolitan  Museum  of 
Art  of  New  York  has  been  conducting  operations  at  this 
magnificent  tomb  of  Seti  I.  The  photographs  of  the  tomb 
examined  with  care  give  glimpses  of  the  glories  of  the  tomb 
and  show  some  of  the  decorations  which  include  the  Lion 
surrounded  by  stars  and  the  Bull.  A  careful  scrutiny  of 
photographs  of  this  and  other  tombs  appears  to  show  that 
the  stars  connected  with  the  Lion  and  the  Bull  in  no  way 
refer  to  the  well-known  zodiacal  constellations  of  Leo  and 
Taurus.  In  fact,  the  signs  of  the  zodiac  were  not  acquired 
from  the  Chaldeans  until  fairly  late  in  Egyptian  history, 
for  there  is  no  record  of  them  until  after  the  Ptolemaic 
period.  If  the  Grand  Pyramid  had  been  in  Babylon  instead 
of  Egypt  it  is  safe  to  say  that  it  would  have  been  used  as  an 
astronomical  observatory.  The  Egyptians  were  not  inter- 
ested in  systematized  observations,  and  throughout  the  long 
years  of  their  interesting  history  they  contributed  but  little 
to  the  science  of  astronomy. 


CHAPTER   II 

BIBLICAL   AND    CLASSICAL   ECLIPSES 

IT  IS  safe  to  say  that  there  is  certainly  one  allusion  in 
the  Holy  Scriptures  to  a  solar  eclipse,  with  one  or  two 
others  possible.  In  Amos  viii,  9,  appear  the  words, 
"  I  will  cause  the  sun  to  go  down  at  noon,  and  I  will  darken 
the  Earth  in  the  clear  day."  The  language  is  so  unmistak- 
able and  gives  such  a  precise  description  of  an  eclipse  of  the 
sun  that  commentators  have  generally  agreed  that  such  a 
phenomenon  must  have  taken  place.  This  and  other  Scrip- 
tural references  have  been  so  fully  and  so  carefully  inves- 
tigated by  Chambers  in  The  Story  of  Eclipses,  and  by  John- 
son, Eclipses,  Past  and  Future,  that  a  brief  reference  only 
will  be  given  here. 

The  date  set  down  in  the  margin  of  certain  Bibles  oppo- 
site this  passage  in  Amos  is  787  B.C.  Eclipses  were  visible 
in  the  neighborhood  of  Samaria  in  the  years  791  B.C.,  771 
B.C.,  770  B.C.  and  763  B.C.  This  last  mentioned  is  the  eclipse 
of  Nineveh  already  described,  an  eclipse  which  was  visible 
also  in  Samaria.  There  seems  no  doubt  whatever  that  this 
is  the  eclipse  predicted  by  Amos.  It  therefore  becomes 
necessary  to  lower  by  twenty-four  years  the  date  given  in 
Amos;  and  if  this  is  done  there  is  a  ready  explanation  of 
the  story  in  the  life  of  King  Hezekiah,  as  given  in  II  Kings 
xx,  ii.  The  Old  Testament  thus  reads:  "  And  Isaiah  the 
prophet  cried  unto  the  Lord:  and  he  brought  the  shadow 
ten  degrees  backward,  by  which  it  had  gone  down  in  the 
dial  of  Ahaz."  Ahaz  was  the  father  of  Hezekiah,  and  the 
"  dial  "  was  probably  a  sun-dial  similar  in  construction  to 
the  ancient  sun-dials  of  masonry  still  existing  at  Benares 
and  Delhi  in  India. 

The  Biblical  peoples  at  this  period  of  their  history  were 
divided  into  two.  In  the  North,  the  land  was  rich  and  fer* 

27 


28  ECLIPSES    OF    THE    SUN 

tile  and  great  prosperity  and  wealth  abounded.  The  people 
Hved  in  large  towns,  they  dressed  and  lived  extravagantly. 
Moreover  these  people  of  Israel  had  adopted  the  gods  of  the 
Canaanites,  each  town  in  fact  having  its  own  god,  or  "  baal." 
To  the  South,  the  land  was  poor,  encroaching  as  it  did  on  the 
desert,  and  the  people  of  Judah  had  to  struggle  for  their 
existence.  The  only  large  town  was  Jerusalem.  The  Jews 
were  still  faithful  to  the  Hebrew  God,  Jehovah.  The  shep- 
herd Amos  being  a  devout  follower  of  Jehovah  made  a 
journey  to  Israel,  and  thundered  his  denunciations  against 
their  many  gods,  and  against  their  gaudy  clothes,  their  licen- 
tiousness and  harsh  treatment  of  the  poor. 

About  this  time  the  Hebrews  were  beginning  to  learn  to 
write,  and  they  were  abandoning  the  clay  tablet  and  the 
cuneiform  inscription  of  the  Assyrians  and  Babylonians. 
They  wrote  on  papyrus  and  with  pen  and  ink,  which  method 
of  writing  they  had  acquired  from  their  former  masters,  the 
Egyptians.  However,  they  borrowed  the  first  alphabet  the 
world  had  developed,  that  of  Phoenicia.  The  papyrus  rolls 
written  by  Amos  and  other  historians  have  descended  to  us 
as  the  Hebrew  Scriptures.  On  account  of  the  evil  lives  of 
the  people  of  Israel,  Amos  predicted  their  destruction. 
Damascus  was  taken  by  the  powerful  Assyrians  in  732  B.C., 
and  thus  being  unprotected  on  the  north,  Samaria,  the  capi- 
tal of  the  kingdom  of  Israel,  was  captured  in  722  B.C.,  the 
Israelites  being  taken  away  captive  and  their  nation  de- 
troyed. 

Shortly  before  700  B.C.  arose  the  great  prophet  Isaiah. 
In  one  great  oration  after  another  he  exhorted  the  people  of 
Jerusalem  to  believe  in  their  God,  Jehovah,  and  not  be  dis- 
mayed for  He  would  deliver  them  from  the  hosts  of  the 
powerful  Assyrian,  Sennacherib,  who  was  then  threatening 
to  batter  down  the  city.  The  king  of  Jerusalem,  Hezekiah, 
was  sick  unto  death,  and  the  same  tragic  fate  as  had  befallen 
Damascus  and  Samaria  seemed  to  be  awaiting  the  people  of 
Jerusalem.  In  answer  to  a  prayer  of  Hezekiah  for  recovery 
the  prophet  was  sent  to  him  with  this  message:  "  Thus 
saith  the  Lord,  the  God  of  David  thy  Father,  I  have 
heard  thy  prayer,  I  have  seen  thy  tears:  behold  I  will  add 


BIBLICAL    AND    CLASSICAL    ECLIPSES  29 

unto  thy  days  fifteen  years  .  .  .  and  I  will  defend  this  city, 
and  this  shall  be  a  sign  unto  thee  from  the  Lord,  that  the 
Lord  will  do  this  thing  that  He  hath  spoken.  Behold,  I  will 
bring  again  the  shadow  of  the  Sun,  which  is  gone  down  in 
the  sun-dial  of  Ahaz  ten  degrees  backward,  by  which  degrees 
it  had  gone  down."  (Isaiah  xxxviii,  5-8.) 

There  was  a  large  partial  eclipse  of  the  sun  visible  in 
Jerusalem  on  January  n,  689  B.C.  If  we  accept  the  correc- 
tion of  twenty-four  years  derived  from  the  record  of  the 
eclipse  of  Nineveh,  then  according  to  Chambers  (loc.  cit.\ 
this  eclipse  of  Jerusalem  will  completely  satisfy  the  Biblical 
narrative  at  all  points.  History  tells  that  a  pestilence  from 
the  marshes  of  the  Nile  caused  great  havoc  in  the  army  of 
Sennacherib,  and  thus  was  Jerusalem  miraculously  spared. 
See  also  in  this  connection,  II  Kings,  xix,  32-37.  About  a 
century  later  the  Hebrews  rejoiced  over  the  destruction  of 
Nineveh  in  606  B.C.  But  unfortunately,  the  days  of  Jeru- 
salem were  numbered.  The  Chaldeans  followed  Assyria  as 
masters  of  Palestine,  Jerusalem  was  destroyed  in  586  B.C. 
under  Nebuchadnezzar,  and  the  people  were  carried  away 
into  exile  in  Babylon. 

The  dates  herewith  given  in  Babylonian  and  Assyrian 
history  admit  of  no  uncertainty  since  they  are  determined 
by  eclipses.  As  a  consequence,  the  dates  appearing  in  the 
Bible  must  be  altered  to  fit  those  of  verified  history.  It 
would  be  interesting  to  follow  the  history  of  the  Hebrews 
through  the  lives  of  Jeremiah  and  other  prophets  —  but 
space  forbids.  The  reader  is  referred  to  Breasted,  Ancient 
Times,  a  History  of  the  Early  World,  and  also  to  many 
other  books  on  this  period  of  history  which  is  of  the  greatest 
of  interest  to  all  Christian  peoples. 

ECLIPSES  OF  CLASSICAL  LITERATURE 

It  will  be  possible  to  give  an  account  here  of  but  a  few  of 
the  eclipses  referred  to  in  the  Classics,  and  we  shall  begin 
with  Homer.  On  the  day  of  the  slaughter  of  the  suitors, 
there  is  a  passage  in  the  Odyssey  (v.  351-357),  which  prob- 
ably refers  to  an  eclipse  of  the  sun.  The  lines  run: 


30  ECLIPSES    OF    THE    SUN 

"  Ah,  wretched  men,  what  evil  is  this  that  you  suffer? 
Shrouded  in  night  are  your  heads  and  your  faces  and  your 
knees  beneath  you;  kindled  is  the  sound  of  wailing,  bathed 
in  tears  are  your  cheeks,  and  sprinkled  with  blood  are  the 
walls  and  the  fair  rafters.  And  full  of  ghosts  is  the  porch 
and  full  the  court,  of  ghosts  that  hasten  down  to  Erebus 
beneath  the  darkness,  and  the  Sun  has  perished  out  of 
heaven,  and  an  evil  mist  hovers  over  all." 

According  to  Fotheringham  (loc.  aY.),  the  words,  "  The 
Sun  has  perished  out  of  heaven/'  must  refer  to  a  total 
eclipse,  and  in  fact  most  commentators,  including  Plutarch 
and  Eustathius,  agree  in  this  interpretation.  Apparently, 
the  eclipse  was  contained  in  the  legend  as  it  descended  to 
Homer,  —  though  naturally  it  is  impossible  to  fix  the  exact 
date  of  the  eclipse. 

The  next  eclipse  is  one  referred  to  by  the  Greek  poet 
Archilochus,  a  portion  only  of  whose  work  has  come  down 
to  us.  The  lines  are:  "  Zeus,  the  father  of  the  Olympic 
Gods,  turned  mid-day  into  night,  hiding  the  light  of  the 
dazzling  Sun;  and  sore  fear  came  upon  men."  According 
to  the  investigations  of  Oppolzer  and  Cowell,  the  poet 
must  have  witnessed  the  eclipse  of  April  6,  648  B.C.,  the 
eclipse  being  total  about  10  A.M.  at  Thasos  and  in  the  north- 
ern part  of  the  Aegean  Sea.  We  know  that  the  poet  spent 
part  of  his  time  at  Thasos,  and  if  his  statement  actually 
gives  the  description  of  an  eclipse,  it  thus  furnishes  the 
earliest  date  in  Grecian  chronology  to  be  fixed  with  cer- 
tainty. The  early  accepted  dates  must  accordingly  be  re- 
duced by  fifty  years,  but  no  surprise  need  be  felt  as  they 
were  known  only  with  great  uncertainty. 

ECLIPSE  OF  THALES 

The  most  celebrated  eclipse  of  all  history  is  that  con- 
nected with  the  name  of  Thales  of  Miletus  who  lived  from 
640  to  546  B.C.  He  was  the  founder  of  Greek  astronomy, 
geometry  and  philosophy.  He  was  regarded  by  the  Greeks 
with  great  veneration,  and  he  was  the  chief  of  the  seven 
"  wise  men,"  a  reputation  which  rested  not  only  on  his 


BIBLICAL    AND    CLASSICAL    ECLIPSES  31 

scientific  eminence  but  also  on  his  political  sagacity.  Ac- 
cording to  the  historian  Herodotus  (i,  74),  the  reference  to 
the  eclipse  is  as  follows:  "  There  was  war  between  the 
Lydians  and  the  Medes  for  five  years,  each  won  many  vic- 
tories from  the  other,  and  once  they  fought  a  battle  by 
night.  They  were  still  warring  with  equal  success,  when  it 
chanced,  at  an  encounter  which  happened  in  the  sixth  year, 
that  during  the  battle  the  day  was  turned  into  night.  Thales 
of  Miletus  had  foretold  this  loss  of  daylight  to  the  lonians, 
fixing  it  within  the  year  in  which  the  change  did  indeed  hap- 
pen. So  when  the  Lydians  and  Medes  saw  the  day  turned 
to  night  they  ceased  from  fighting,  and  both  were  the  more 
zealous  to  make  peace."  The  truce  concluded  was  cemented 
by  a  double  marriage,  "  for/'  adds  the  historian,  "  without 
some  such  strong  bond,  there  is  little  of  security  to  be  found 
in  men's  covenants."  The  same  eclipse  is  referred  to  by 
both  Pliny  and  Cicero. 

The  narrative  by  Herodotus  contains  two  statements, 
that  the  eclipse  was  total  (a  "  night  battle  "),  and  that  the 
eclipse  was  predicted  by  Thales.  It  is  in  regard  to  the 
second  fact  that  the  controversy  has  arisen  among*  astrono- 
mers. Various  dates  for  the  eclipse  have  been  assigned, 
from  625  B.C.  to  583  B.C.  But  after  careful  researches  of 
many  competent  authorities  —  Airy,  Hind,  Zech,  Hansen, 
Ginzel,  Newcomb,  Cowell,  Fotheringham  and  others  —  the 
date  of  May  28,  585  B.C.,  has  been  fixed,  the  eclipse  taking 
place  in  the  afternoon.  The  general  consensus  of  opinion  is 
that  Thales  was  probably  familiar  with  the  Chaldean  Saros 
of  223  lunar  months  or  18  years  n  days.  In  fact,  by  some  it 
has  been  thought  (see,  George  Smith,  Assyrian  Discoveries} 
that  the  Babylonians  were  actually  making  use  of  the  Saros 
for  the  prediction  of  eclipses.  Herodotus  does  not  claim  that 
the  day  and  hour  of  the  eclipse  were  predicted  nor  yet  the 
locality  where  it  was  to  be  visible,  these  being  much  more 
difficult  problems  and  incapable  of  being  solved  by  Thales. 
The  eclipse  was  predicted  only  "  within  the  year."  It  is 
therefore  not  necessary  to  credit  Thales  with  extraordinarily 
abnormal  powers,  or  to  assume  that  he  was  in  possession  of 
information  which  was  known  first  to  Hipparchus  more  than 


32  ECLIPSES    OF    THE    SUN 

four  hundred  years  later.  Apparently  the  Greek  "  wise 
man  "  utilized  the  eclipse  of  May  17,  603  B.C.,  for  the  pur- 
pose of  his  prediction. 

Probably  the  greatest  difficulty  of  all  concerning  the  iden- 
tification of  this  eclipse  of  Thales  has  been  to  know  just  how 
much  credence  could  be  placed  in  the  account  of  Herodotus. 
Some  of  the  ancient  writers  go  so  far  as  to  accuse  him  of 
"  intentional  untruthfulness."  Modern  critics  while  not  go- 
ing to  this  extreme,  none  the  less  feel  that  his  love  of  effect 
and  his  loose  and  inaccurate  habits  of  thought  make  of  him 
an  attractive  writer,  but  a  poor  historian.  A  case  in  point 
might  be  cited,  the  eclipse  which  is  described  by  Herodotus 
(vii,  37)  as  follows:  "  At  the  first  approach  of  spring,  the 
army  quitted  Sardis  and  marched  towards  Abydos;  at  the 
moment  of  its  departure  the  Sun  quitted  its  place  in  the 
heavens  and  disappeared,  though  there  were  no  clouds  in 
sight  and  the  day  was  quite  clear;  day  was  thus  turned  into 
night. "  We  are  told  that  "  As  the  King  was  going  against 
Greece,  and  had  come  into  the  region  of  the  Hellespont, 
there  happened  an  eclipse  of  the  Sun  in  the  East;  this  sign 
portended  to  him  his  defeat  for  the  sun  was  eclipsed  in  the 
region  of  his  rising,  and  Xerxes  was  also  marching  from  that 
quarter."  The  generally  accepted  date  of  the  battles  of 
Thermopylae  and  Salamis  is  480  B.C.  No  eclipse  occurred  in 
the  spring  of  that  year.  Many  attempts  have  been  made  to 
find  an  eclipse  that  would  harmonize  with  the  statement  of 
Herodotus,  but  all  of  no  avail,  —  unless  we  are  willing  to 
make  changes  either  in  the  date  or  in  the  narrative.  It 
seems  necessary  to  conclude  that  the  eclipse  incident  adds 
greatly  to  the  attractiveness  of  the  story,  —  but  that  it  is 
not  history.  Unfortunately,  even  at  the  present  day, 
authors  still  try  to  embellish  their  stories  with  celestial 
phenomena,  but  do  not  stick  too  closely  to  the  facts.  One 
of  the  "  best  sellers  "  appearing  recently  in  the  field  of  fic- 
tion had  in  it  no  less  than  four  astronomical  blunders. 

The  next  eclipse  to  be  mentioned  was  seen  in  Athens  in 
B.C.  431,  August  3.  It  is  described  by  Thucydides  (ii?  28) 
who  states  that  during  the  Peloponnesian  War  "  things  for- 
merly repeated  on  hearsay,  but  very  rarely  confirmed  by 


BIBLICAL    AND    CLASSICAL    ECLIPSES  33 

facts,  became  not  incredible,  both  about  earthquakes  and 
eclipses  of  the  Sun  which  came  to  pass  more  frequently  than 
had  been  remembered  in  former  times."  An  eclipse  occurred 
in  the  first  year  of  the  war,  "  in  the  same  summer,  at  the 
beginning  of  a  new  lunar  month  (at  which  time  alone  the 
phenomenon  seems  possible),  the  Sun  was  eclipsed  after 
midday,  and  became  full  again  after  it  had  assumed  a  cres- 
cent form  and  after  some  of  the  stars  had  shone  out"  The 
account,  differing  from  the  flowery  but  uncertain  language 
of  Herodotus,  is  clear  and  definite,  and  refers  to  an  eclipse 
which  evidently  was  not  total  at  Athens  where  Thucydides 
was  supposed  to  be.  The  only  difficulty  in  fixing  the  eclipse 
has  arisen  from  the  last  part  of  the  statement,  that  stars 
were  visible.  Venus  was  10°  distant  and  undoubtedly  must 
have  been  readily  seen,  but  what  other  star,  or  stars?  The 
eclipse  in  B.C.  431  was  seven-eighths  total.  To  make  a  long 
story  short,  it  sufficeth  to  say  that  at  the  eclipse  of  April  8, 
1921,  which  was  eight-ninths  total  at  Oxford,  the  star  Vega 
was  readily  seen.  It  must  be  concluded,  therefore,  that  the 
account  of  Thucydides  is  strictly  in  accord  with  the  facts 
and  that  the  stars  Venus,  Vega  and  probably  Jupiter  43° 
distant  were  seen. 

This  same  eclipse  has  an  interesting  anecdote  connected 
with  it  which  has  been  narrated  by  Plutarch  in  his  Life  of 
Pericles  who  was  the  commander  of  the  Grecian  naval 
forces.  "  The  whole  fleet  was  in  readiness,  and  Pericles  on 
board  his  own  galley  when  there  happened  an  eclipse  of  the 
Sun.  The  sudden  darkness  was  looked  upon  as  an  unfavor- 
able omen,  and  threw  the  sailors  into  the  greatest  consterna- 
tion. Pericles,  observing  that  the  pilot  was  much  astonished 
and  perplexed,  took  his  cloak,  and  having  covered  his  eyes 
with  it,  asked  him  if  he  found  anything  terrible  in  that,  or 
considered  it  a  bad  presage?  Upon  his  answering  in  the 
negative,  he  asked  '  Where  is  the  difference,  then,  between 
this  and  the  other,  except  something  bigger  than  my  cloak 
causes  the  difference?  '  " 

Another  eclipse  recorded  by  Thucydides  (iv,  52)  has  been 
identified  with  the  annular  eclipse  of  March  21,  424  B.C. 

The  last  of  the  eclipses  described  by  Thucydides  (vii  50) 


34  ECLIPSES    OP    THE    SUN 

is  one  of  the  moon,  and  it  has  a  tragic  story  connected  with 
it.  The  Athenian  fleet  and  army  were  engaged  in  an  attack 
on  Syracuse,  the  fleet  being  under  the  command  of  Nicias, 
and  Demosthenes  had  arrived  with  large  reinforcements 
for  the  army.  The  latter  had  failed  in  his  purpose  of  cap- 
turing a  wall  which  the  Syracusians  had  thrown  across  the 
Athenian  lines,  and  as  a  result  of  this  failure  it  had  been 
decided  to  withdraw  the  whole  Athenian  forces.  On  the 
night  of  August  27,  413  B.C.,  the  whole  force  had  embarked 
and  was  ready  to  flee.  Before  a  start  was  made,  however, 
an  eclipse  of  the  moon  took  place  which  became  total. 
Nicias  was  greatly  terrified,  for  it  seemed  easy  to  under- 
stand the  cause  of  an  eclipse  of  the  sun,  but  very  difficult 
to  make  out  "  how  the  moon,  when  at  the  full,  should  sud- 
denly lose  her  light  and  assume  such  a  variety  of  colors." 
The  soldiers  and  sailors  added  their  terror  to  that  of  the 
commander  and  as  a  result  the  soothsayers  were  consulted. 
The  advice  was  to  remain  for  thrice  nine  days,  —  but  alas, 
before  that  time  the  Athenians  had  been  utterly  routed,  and 
Nicias  and  Demosthenes  were  both  executed. 

Plutarch  records  an  eclipse  of  the  sun  which  took  place 
July  13,  364  B.C.,  and  both  Plutarch  and  Pliny  give  an  ac- 
count of  a  total  eclipse  of  the  moon  September  20,  331  B.C., 
eleven  days  before  the  victory  of  Alexander  over  Darius  at 
Arbela  in  Assyria. 

Another  interesting  eclipse  of  the  sun  is  connected  with 
the  name  of  Agathocles,  the  tyrant  of  Syracuse.  The  narra- 
tives of  the  two  historians  Justin  and  Diodorus  Siculus  agree 
so  well  that  the  astronomer  has  been  able  to  fix  exactly  the 
circumstance  of  the  eclipse.  In  310  B.C.  Syracuse  was  again 
besieged,  this  time  not  by  Athens,  but  by  Carthage.  On 
the  evening  of  August  14,  Agathocles  slipped  out  of  the  har- 
bor with  his  whole  fleet  of  sixty  ships.  He  was  pursued  by 
the  Carthaginians,  who  abandoned  the  chase  at  night-fall. 
According  to  the  historical  accounts,  there  was  an  eclipse  of 
the  sun  on  the  following  morning,  and  as  a  consequence  day 
was  turned  into  night,  and  stars  appeared  everywhere  in  the 
firmament.  The  soldiers  were  at  first  afraid,  believing  the 
eclipse  to  be  an  ill  omen,  but  their  fears  were  quickly  calmed 


BIBLICAL    AND    CLASSICAL    ECLIPSES  35 

by  their  commander  who,  according  to  Justin,  told  them  that 
eclipses  "  always  signify  a  change  of  affairs,  and  therefore 
some  change  was  certainly  signified,  either  to  Carthage, 
which  was  in  such  a  flourishing  condition,  or  to  them,  whose 
affairs  were  in  a  very  ruinous  state."  Apparently,  these 
words  of  Agathocles  seemed  to  have  inspired  his  men  that 
an  eclipse  might  even  be  an  omen  for  good.  At  any  rate, 
he  was  able  to  make  good  his  escape  from  the  fleet  of  the 
Carthaginians,  and  after  six  days  and  nights  he  succeeded 
in  landing  on  the  coast  of  Africa  and  in  devastating  the 
territories  of  proud  Carthage. 

The  readiness  with  which  Agathocles  and  Pericles  quieted 
the  fears  of  the  sailors  calls  to  mind  the  genius  of  Columbus 
who?  on  more  than  one  occasion,  had  to  quell  mutinies  among 
the  members  of  his  little  fleet,  they  being  greatly  perturbed 
on  discovering  that  the  compass  needle  did  not  point  to  the 
true  north  as  given  by  the  pole  star. 

Mention  of  only  one  other  eclipse  before  the  beginning 
of  the  Christian  era  will  be  made  here.  An  eclipse  is  gen- 
erally regarded  to  have  taken  place  when  Julius  Caesar  was 
crossing  the  Rubicon  on  his  triumphal  return  to  Rome  from 
Gaul.  But  it  seems  to  be  necessary  to  class  this  eclipse 
along  with  that  connected  with  the  name  Xerxes,  —  for  it 
failed  to  occur. 

In  the  investigations  into  the  authenticity  and  accuracy  of 
the  early  eclipses,  the  greatest  difficulty  has  arisen  from  the 
vagueness  and  uncertainties  of  the  account  of  the  chronicler. 
References  to  eclipses  are  couched  in  such  hazy  language 
that  the  account  might  have  been  equally  well  applied  to 
some  other  rare  phenomenon.  Writers  of  literature  in  all 
ages  seem  to  prefer  flowery  to  straightforward  language  and 
employ  imagery  rather  than  fact,  refusing  to  "  call  a  spade, 
a  spade."  For  the  astronomer  it  is  impossible  to  take  one 
interesting  eclipse  and  decide  on  the  pros  and  cons  in  favor 
of  this  or  of  that  interpretation.  Eclipses  depend  on  the 
motions  of  the  sun  and  moon,  and  the  history  of  all  eclipses 
must  make  a  harmonious  story  and  not  a  disjointed  one. 
And  so  the  eclipse  of  Agathocles  must  be  considered  in 
reference  to  that  of  Thales,  and  that  in  turn  to  the  eclipse 


36  ECLIPSES    OF    THE    SUN 

of  Nineveh  in  763  B.C.    The  astronomer  is  not  a  dealer  in 
necromancy  nor  does  he  acquire  information  about  the  mo- 
tions of  the  sun  and  moon  by  mystical  incantations,  or  by 
divination.    The  work  of  the  scientist  is  as  far  removed  as 
possible  from  the  mountebank  and  charlatan.     Scientific 
knowledge  is  acquired  only  by  the  patient  study  of  all  that 
has  been  accomplished  in  the  past,  and  happy  and  fortunate 
is  he  who  can  make  a  correct  interpretation  of  the  past! 
The  manner  in  which  the  story  of  eclipses  has  added  to 
astronomical  knowledge  will  be  treated  in  a  future  chapter. 
Those  who  wish  additional  information  regarding  the  early 
eclipses  would  do  well  to  consult  Encyclopaedia  Britannica, 
article  on  Eclipses  by  Simon  Newcomb,  but  specially  the 
monumental  work  by  Oppolzer,   Canon   der  Finsternisse, 
which  gives  a  record  of  no  less  than  13,200  eclipses,  of  which 
5,200  are  of  the  moon  and  8,000  of  the  sun.    These  eclipses 
are  the  ones  which  have  taken  place  since  1208  B.C.,  over 
three  thousand  years  ago,  or  which  will  take  place  before 
the  end  of  the  year  2162  A.D.,  two  hundred  years  in  the  fu- 
ture.   In  addition  to  the  tables  of  the  astronomical  elements 
of  each  eclipse,  160  charts  are  furnished  giving  the  location 
on  the  earth's  surface  where  each  total  and  annular  eclipse 
of  the  sun  will  be  visible.     It  should,  however,  be  pointed 
out  that  the  eclipse  tracks  could  not  be  located  by  Oppolzer 
on  the  maps  with  the  greatest  of  refinement,  since  it  was 
manifestly  impossible  to  calculate  the  location  of  each  path 
for  more  than  three  positions,  sunrise,  noon  and  sunset. 
Consequently,  it  need  not  be  a  matter  of  surprise,  par- 
ticularly with  the  early  eclipses,  that  the  tracks  may  at  times 
be  incorrectly  placed  by  as  much  as  a  hundred  miles. 


CHAPTER   III 

THE   PREDICTION    OF    ECLIPSES 

AFTER  the  destruction  of  Babylonia  and  Assyria, 
astronomy  migrated  from  its  Chaldean  home  to 
Greece.  As  early  as  the  fifth  century  B.C.,  attempts 
were  made  to  demonstrate  some  of  the  practical  uses  of 
astronomy  in  the  regulation  of  time  and  particularly  in  the 
arranging  of  the  calendar.  The  Greeks  had  inherited  from 
the  Chaldeans  a  calendar  founded  on  the  lunar  month,  but 
since  the  month  is  29.5  days  in  length,  attempts  to  bring 
the  seasons  and  the  calendar  into  harmony  ended  always  in 
hopeless  confusion.  The  astronomer  Meton  (born  about 
460  B.C.)  made  the  discovery  that  19  solar  years  were  very 
nearly  equivalent  in  length  to  235  lunar  months  —  the 
difference,  in  fact,  being  only  about  half  an  hour.  The 
Metonic  cycle,  as  it  has  since  been  called,  has  been  of  the 
very  greatest  service  since  its  discovery  so  many  centuries 
ago.  It  should  not  be  confounded  with  the  interval  called 
the  Saros,  since  the  Metonic  cycle  is  not  used  for  the  predic- 
tion of  eclipses.  After  nineteen  years,  new  and  full  moon 
and  the  various  phases  are  repeated  with  a  considerable 
degree  of  exactness.  This  cycle  is  still  used  for  finding  the 
day  on  which  Easter  will  fall. 

The  Chaldeans  had  confined  their  energies  to  the  making 
of  astronomical  observations,  but  their  successors,  the  early 
Greek  philosophers,  made  practically  no  observations  whose 
record  is  worth  keeping.  On  the  other  hand  they  were  much 
interested  in  inquiring  into  the  causes  of  things,  and  ac- 
cordingly it  may  be  said  that  the  science  of  astronomy  had 
its  birth  with  the  Greeks.  We  are  already  familiar  with 
Thales  and  part  of  his  work.  To  Pythagoras  we  owe  the 
doctrine  of  the  "  music  of  the  spheres/'  a  notion  which  has 
descended  even  to  the  present  day.  To  Plato  (428-347 

37 


38  ECLIPSES    OF    THE    SUN 

B.C.)  astronomy  owes  but  little,  he  even  going  so  far  as 
to  decry,  as  degrading,  the  observation  of  the  heavenly 
bodies.  Far  different,  however,  was  the  case  with  Aristotle 
(384-322  B.C.)  who  appears  to  have  collected  and  systema- 
tized the  best  thought  of  the  time  regarding  astronomy. 
He  believed  that  all  of  the  heavenly  bodies,  including  sun, 
earth  and  moon,  were  spherical;  he  taught  that  the  moon 
had  no  light  of  its  own  but  shone  by  reflected  sunlight  and 
he  explained  its  phases.  Most  important  of  these  conclu- 
sions was  the  proof  of  the  spherical  shape  of  the  earth. 
This,  as  Aristotle  demonstrated,  depends  on  two  facts:  first, 
that  the  earth  casts  a  shadow  and  that  an  eclipse  of  the 
moon  is  due  to  the  passing  of  the  moon  into  this  shadow; 
and  second,  that  the  shadow  being  circular  in  outline  was 
proof  that  the  body  casting  the  shadow,  the  earth,  was  cir- 
cular, at  least  in  the  section  turned  towards  the  moon. 

After  the  time  of  Aristotle,  Greek  astronomy  moved  its 
home  to  Alexandria,  and  there  under  the  protection  of  suc- 
cessive Ptolemies  the  great  museum  was  erected  and  a 
library  and  an  observatory  were  incorporated  as  important 
adjuncts.  It  is  not  the  purpose  here  to  trace  the  develop- 
ment of  astronomy  except  as  it  progressed  through  the  study 
of  eclipses.  The  greatest  astronomer  of  antiquity,  in  fact 
the  most  famous  up  to  the  time  of  Newton,  was  Hipparchus. 
He  was  not  of  Alexandria,  though  he  probably  visited  the 
city  and  may  have  made  some  of  his  observations  there. 
Little  is  known  of  his  birth.  This  may  have  taken  place  in 
Bithynia  or  in  Rhodes,  though  it  is  known  that  he  had  an 
observatory  in  the  latter  place  and  there  did  most  of  his 
work.  We  know  of  him  mainly  through  the  astronomer 
Ptolemy,  since  all  of  the  original  works  of  Hipparchus  have 
been  lost  with  the  exception  of  one  important  book.  His 
great  fame  rests  upon  three  pedestals:  (i)  The  invention 
of  trigonometry,  (2)  the  making  of  an  extensive  series  of 
careful  observations,  (3)  the  comparison  of  his  own  with 
earlier  observations.  As  a  result  of  his  exact  and  methodical 
work,  he  discovered  the  precession  of  the  equinoxes,  he 
made  the  first  catalogue  of  the  positions  of  stars  (to  the 
number  of  1080),  and  he  contributed  greatly  to  the  theory 


THE    PREDICTION    OF    ECLIPSES  39 

of  the  motion  of  the  sun  and  rnoon,  and,  as  a  direct  con- 
sequence, to  the  subject  of  eclipses. 

Hipparchus  was  the  first  to  notice  that  the  seasons  were 
of  unequal  length,  the  time  from  vernal  equinox  to  summer 
solstice  being  94  days,  while  the  summer  season  was  only 
92 1  days  in  length.  Spring  and  summer  together  are  seven 
days  longer  than  fall  and  winter.  Hipparchus  gave  the 
correct  explanation  of  this  fact,  viz:  that  the  earth  is  not 
always  at  a  constant  distance  from  the  sun,  being  removed 
from  the  center  of  the  orbit  (which  was  then  regarded  as 
circular).  At  the  present  day  this  is  readily  verified  by  the 
change  in  the  angular  diameter  of  the  sun,  —  but  this  con- 
firmation in  Hipparchus'  time  would  have  required  measure- 
ments too  exact  for  his  crude  instruments.  By  noting  the 
time  it  takes  the  moon  to  pass  through  the  earth's  shadow 
at  the  time  of  a  total  eclipse  of  the  moon,  Hipparchus  was 
able,  by  a  method  due  to  Aristarchus,  to  obtain  a  value 
of  the  relative  sizes  of  earth  and  moon,  and  also  to  esti- 
mate the  distance  of  the  moon  to  be  59  times  the  terrestrial 
radius,  —  which  is  not  very  far  from  the  truth. 

The  motion  of  the  moon  was  much  more  difficult  to  in- 
vestigate than  that  of  the  sun.  Little  observation  is  neces- 
sary to  note  that  the  moon  moves  eastward  in  the  sky, 
changing  her  position  among  the  stars  by  her  own  diameter 
in  approximately  an  hour.  A  revolution  of  the  moon  from 
a  star  to  the  same  star  again  is  known  as  a  sidereal  month, 
and  its  length  is  27.3  days.  In  this  interval  the  earth  has 
proceeded  in  its  orbit  about  the  sun,  and  consequently  the 
moon,  as  seen  from  the  sun,  requires  more  than  two  ad- 
ditional days  to  swing  into  line  with  the  earth.  The  in- 
terval of  time  from  new  moon  to  new  moon,  or  from  full 
moon  to  full  moon,  is  the  ordinary  month  of  29.5  days,  called 
the  synodic  month.  Further  observation  shows  that  the 
sun  is  always  in  the  ecliptic,  in  fact,  the  sun's  apparent  mo- 
tion determines  the  ecliptic,  whereas  the  motion  of  the 
moon,  although  approximately  in  the  ecliptic,  is  neverthe- 
less inclined  at  a  small  angle,  which  Hipparchus  for  the 
first  time  fixed  as  at  an  angle  of  5°.  The  moon's  path  thus 
crosses  the  ecliptic  at  two  points  called  the  nodes.  If  the 


40  ECLIPSES    OF    THE    SUN 

moon's  path  were  exactly  in  the  plane  of  the  ecliptic  there 
would  be  two  eclipses  each  month,  once  at  the  time  of  new 
moon  when  the  moon  would  come  between  the  earth  and 
the  sun,  bringing  about  an  eclipse  of  the  sun;  and  again  at 
full  moon,  when  the  moon  would  pass  into  the  shadow  cast 
by  the  earth  and  be  itself  eclipsed.  But  as  the  moon's  orbit 
is  inclined  to  the  ecliptic,  it  is  manifest  that  eclipses,  either 
of  sun  or  moon,  can  occur  only  when  the  moon  is  near  the 
plane  of  the  ecliptic,  or,  in  other  words,  near  its  node.  If 
the  length  of  time  it  takes  the  moon  to  return  to  its  node 
were  the  same  as  the  sidereal  month,  it  is  evident  that  the 
moon's  node  would  be  fixed  in  space  and  would  have  no 
motion  relative  to  the  stars.  The  length  of  the  month  deter- 
mined by  the  return  of  the  moon  to  the  node  is  called  the 
nodical,  or  the  draconic  month.  The  meaning  of  the  first 
designation  is  evident.  But  why  the  second?  It  is  not  diffi- 
cult to  trace  it  back  to  the  ancient  belief  that  the  sun  was 
swallowed  by  a  dragon  at  the  time  of  an  eclipse,  and  this 
superstition  is  therefore  the  reason  why  the  month  on 
which  eclipses  depend  should  be  called  draconic.  Indeed 
the  symbols  that  are  universally  used  by  astronomers  to  de- 
note the  ascending  node  (  & )  and  the  descending  node  (  W  ) 
are  generally  supposed *  to  represent  the  head  and  tail  of  the 
dragon.  Hipparchus  found  that  the  moon's  nodes  were  not 
fixed,  but  that  they  completed  a  revolution  in  the  plane  of 
the  ecliptic  from  east  to  west  in  about  19  years.  A  fourth 
kind  of  month  was  likewise  known  to  Hipparchus,  the  length 
of  a  revolution  from  the  position  of  perigee  to  perigee  in  the 
moon's  orbit.  By  making  use  of  the  eclipses  observed  by 
the  Chaldeans,  Hipparchus  was  enabled  to  obtain  greatly 
improved  values  of  the  lengths  of  the  various  months.  The 
values  that  follow  are  those  furnished  by  the  recent  investi- 
gations of  E.  W.  Brown: 


Synodical      = 

29.0530588    - 

2Qd 

I2h 

44rn 

2. 

S8 

Sidereal         = 

27. 

321661  = 

27 

7 

43 

II. 

5 

Anomalistic  = 

27. 

554550  = 

27 

13 

18 

33- 

i 

Nodical         = 

27. 

2I222O    = 

27 

5 

5 

35- 

8 

See  Berry,  A  Short  History  of  Astronomy,  p.  48. 


THE    PREDICTION    OF    ECLIPSES  41 

The  lengths  of  the  different  kinds  of  year  will  be  inserted 
here  for  future  reference,  the  values  being  Newcomb's: 

Tropical  (ordinary)   =  365^242199 
Eclipse  =  346.  620031 

Since  the  motion  of  the  moon's  node  is  in  the  direction 
to  meet  the  oncoming  sun,  the  interval  of  time  from  node 
to  node  (the  eclipse  year)  is  less  than  the  interval  from 
vernal  equinox  to  vernal  equinox,  the  ordinary  year  on 
which  the  seasons  depend.  From  the  above  figures,  the 
eclipse  year  is  18.62  days  shorter  than  the  tropical  year. 
Since  the  sun  and  earth  are  always  in  the  plane  of  the 
ecliptic,  all  that  is  required  to  permit  the  prediction  of  an 
eclipse  of  the  sun  is  to  find  at  the  time  of  the 
new  moon  (when  alone  a  solar  eclipse  can  take  place) 
whether  the  moon  is  sufficiently  near  enough  to  the  plane 
of  the  ecliptic  for  her  to  come  between  earth  and  sun.  And 
as  an  eclipse  of  the  moon  can  happen  only  at  full  moon, 
similar  investigations  will  furnish  the  means  of  predicting 
lunar  eclipses.  With  the  modern  values  given  above,  these 
predictions  can  be  carried  out  remarkably  easily.  Let  us 
see: 

19  eclipse  years  =  6585^7806 

223  ordinary  months  =  6585.  3211 
247  nodical  months  =  6585.  3572 
239  anomalistic  months  =  6585.  5374 

The  integral  parts  of  these  four  quantities  are  the  same, 
the  second  being  the  Saros  (meaning  "  repetition  ")  which 
amounts  to  18  years  n  days,  if  only  four  leap  years  inter- 
vene, or  1 8  years  10  days  if  the  2gth  of  February  has  come 
five  times.  The  eclipse  of  June  8,  1918  was  a  repetition 
of  that  of  May  28,  1900;  the  eclipse  of  September  10,  1923 
followed  that  of  August  30,  1905  by  the  interval  of  one 
Saros.  The  author  observed  the  1900  eclipse  in  Georgia 
and  that  of  1918  in  Oregon.  It  was  necessary  for  him  to 
travel  to  Spain  to  witness  the  1905  eclipse  and  to  California 
for  the  one  of  1923.  Succeeding  eclipses  in  the  Saros  are 
visible  each  time  from  a  location  on  the  earth's  surface 
farther  west.  The  reason  for  this,  and  the  causes  of 
further  differences  in  eclipses  which  follow  each  other  are 


42 


ECLIPSES    OF    THE    SUN 


readily  explained  from  the  fact  that  the  numbers  given 
above  are  not  exactly  equal  to  each  other  but  differ  in  the 
decimal  parts.  Before  going  into  this  in  detail,  it  will  be 
well  to  take  up  some  of  the  geometrical  features  useful  in 
the  prediction  of  eclipses. 


FIG.  i 

The  length  of  the  shadow  cast  by  earth  or  moon  can  be 
readily  found.  S  is  the  center  of  the  sun,  E  that  of  the 
earth,  and  M  and  m  the  moon's  center  at  new  and  full 
moon,  respectively.  A  shadow  cone  envelopes  the  sun  and 
earth,  its  vertex  being  at  T.  If  the  moon  in  her  orbit  passes 
wholly  into  the  shadow  there  is  a  total  eclipse  oj  the  moon. 
If  the  line  ED  is  drawn  parallel  to  TEA,  then  it  is  seen  that 
the  triangle  DSE  and  BET  are  similar,  and  hence 


E  T 
E  B 


S_E 
S  I) 


But  SD  =  SA-AD=R-r,  if  R  and  r  denote  the  radii  of 
sun  and  earth  respectively.  SE  is  the  distance  of  sun  to 
earth  which  equals  A.  Hence  the  length  of  the  shadow, 

4* 

E  T  = 


R-r 

But  R,  the  radius  of  the  sun,  is  equal  to  109.5  times  the 
radius  of  the  earth,  r,  and  accordingly,  the  length  of  the 
earth's  shadow  is  equal  to 

A 


THE    PREDICTION    OF    ECLIPSES  43 

Assuming  the  average  value  of  the  distance  from  sun  to 
earth  as  92,900,000  miles,  then  the  length  of  the  earth's 
shadow  is  on  the  average  857,000  miles,  the  distance  of  the 
earth  to  moon  being  much  less  than  this,  or  238,840  miles. 
In  quite  a  similar  manner  it  is  possible  to  find  the  length 
of  the  shadow  cast  by  the  moon,  by  taking  the  moon  instead 
of  the  earth,  and  assuming  C  as  the  length  of  the  moon's 
radius,  and  D  the  distance  from  centers  of  sun  to  moon. 
The  length  of  the  shadow  cast  by  the  moon  is  equal 


R-C 


D 


But  R  and  C?  the  radii  of  the  sun  and  moon,  are  each  con- 
stant, and  substituting  the  values  it  is  found  that  the  length 
of  the  moon's  shadow  is  3-9^75-5  of  the  moon's  distance  from 
the  sun,  or  approximately  one  four-hundredth  part  of  the 
distance.  Under  the  conditions  given  a  total  eclipse 
of  the  sun,  the  distance  from  sun  to  moon,  D,  is 
equivalent  to  the  difference  between  the  distances  from 
sun  to  earth  and  from  earth  to  moon.  On  account  of 
the  fact  that  the  orbits  of  earth  and  moon  are  both 
ellipses  and  not  circles,  the  distance  D  will  vary  consider- 
ably and  the  length  of  the  moon's  shadow  will  change  pro- 
portionally. Under  average  conditions,  the  length  of  the 
shadow  measured  from  the  center  of  the  moon  is  231,650 
miles,  but  this  may  vary  about  4000  miles  each  way  from 
the  mean,  or  from  235,700  miles  as  a  maximum  to  228,120 
miles  as  a  minimum.  Since  the  mean  distance  from  center 
of  earth  to  center  of  moon  is  238,840  miles,  or  from  moon's 
center  to  earth's  surface  234,900  miles,  it  is  seen  that  under 
average  conditions  the  moon's  shadow  is  not  long  enough 
to  reach  the  earth's  surface.  The  distance  D  will  have  its 
greatest  value  when  the  earth  is  at  its  greatest  distance  from 
the  sun,  at  aphelion,  which  takes  place  about  July  i  of  each 
year,  and  the  moon  at  its  least  distance  from  the  earth, 
or  at  perigee.  Owing  to  the  revolution  of  the  moon's  line  of 
apsides,  perigee  may  come  during  any  month  of  the  year. 
The  distance  D  is  least  when  the  earth  is  at  perihelion 
(January  i)  and  the  moon  at  apogee.  On  account  of  the 


44  ECLIPSES    OF    THE    SUN 

elliptical  character  of  the  moon's  orbit,  the  distance  from 
center  of  earth  to  center  of  moon  varies,  from  approximately 
253,000  miles  as  a  maximum  to  221,600  miles  as  a  min- 
imum. The  latter  distance  is  217,650  miles  from  the  earth's 
surface.  Since  the  shadow  cast  by  the  moon  may  be  235,700 
miles  in  length,  its  vertex  therefore  at  times  may  extend 
18,000  miles  beyond  the  earth's  surface.  Under  these  con- 
ditions the  area  of  the  moon's  shadow  cone  intercepted  by 
the  earth  will  be  a  maximum,  and  when  all  conditions  are 
most  favorable,  the  diameter  will  be  163  miles.  Such  condi- 
tions are  possible  when  the  total  eclipse  is  visible  about  July 
i,  with  the  moon  at  perigee,  the  eclipse  being  visible  at  the 
earth's  equator  at  noon.  The  diameter  of  the  moon's 
shadow  path  intercepted  by  the  earth  may  vary  from  its 
maximum  value  to  a  vanishing  width  when  the  shadow  is 
just  long  enough  to  reach  the  earth's  surface.  The  average 
width  is  approximately  one  hundred  miles.  Inside  this 
shadow  cone  an  observer  will  see  the  light  of  the  sun  totally 
cut  off;  outside  of  it  the  sun  will  be  partially  eclipsed. 

Since  also  the  distance  may  be  as  great  as  253,000  miles 
from  the  earth's  center,  or  about  249,000  miles  from  its 
surface,  while  the  moon's  shadow  may  be  only  228,120  miles 
in  length,  the  shadow  may  fall  short  of  the  earth's  surface  by 
more  than  20,000  miles.  Under  these  conditions  an  ob- 
server on  the  earth  located  on  the  axis  of  the  shadow  pro- 
duced, would  see  an  annular  eclipse  and  not  a  total  one,  a  ring 
or  annulus  of  light  appearing  round  the  edge  of  the  moon 
when  the  eclipse  is  central.  The  diameter  of  the  so-called 
negative  shadow  may  be  as  great  as  230  miles.  Since  the 
maximum  width  of  the  shadow  causing  a  total  solar  eclipse 
is  only  163  miles,  it  is  evident  that  the  number  of  annular 
eclipses  are  more  frequent  than  total  —  only  two  out  of 
five  central  eclipses  being  total. 

The  work  of  the  observational  astronomer  has  been 
greatly  hampered  by  the  moon  which  illuminates  the  sky 
and  renders  it  impossible  to  see  or  to  photograph  the  faint- 
est stars  in  the  telescope.  As  already  has  been  noted,  the 
moon  has  furnished  much  labor  and  vexation  of  spirit  to 
the  mathematical  astronomer.  Yet  if  our  satellite  were 


THE    PREDICTION    OF    ECLIPSES  45 

banished  from  the  sky,  what  a  dreary  old  world  it  would 
be  to  poets  and  lovers  without  their  "  inconstant  moon  "! 
If  the  diameter  of  the  moon  were  decreased  a  scant  hundred 
and  forty  miles  (less  than  seven  percent),  a  total  eclipse 
of  the  sun  would  be  impossible!  How  thankful  solar  as- 
tronomers should  be  for  the  strange  coincidence  that  the 
angular  diameter  of  the  moon  is  about  equivalent  to  that 
of  the  sun  and  that  the  diameter  of  the  moon  is  not  ten 
percent  less  than  it  is !  The  greatest  possible  excess  of  the 
angular  radius  of  the  moon  over  that  of  the  sun  is  only 
i'  19". 

Knowing  the  diameter  of  the  shadow  intercepted  by  the 
earth  it  is  easy  to  find  the  duration  of  totality.  For  this 
purpose  it  is  necessary  to  find  the  speed  of  the  shadow 
over  the  earth.  The  moon  advances  in  her  orbit  approxi- 
mately her  own  diameter  in  an  hour,  or  more  exactly,  about 
2100  miles  per  hour.  On  account  of  the  great  distance  of 
the  sun,  this  is  the  speed  that  the  moon's  shadow  passes 
across  the  earth  regarded  as  a  whole,  but  on  account  of  the 
rotation  of  the  earth  on  its  axis  the  velocity  of  the  moon's 
shadow  over  the  earth's  surface  is  far  different  from  2100 
miles  per  hour.  The  diurnal  rotation  causes  an  observer 
at  the  equator  to  complete  a  circuit  of  nearly  25,000  miles 
in  twenty-four  hours  which  is  at  the  rate  of  1040  miles  per 
hour.  Away  from  the  equator,  farther  north  and  south  in 
latitude,  the  speed  is  progressively  less  at  greater  and 
greater  distances  from  the  equator,  being  in  fact  1040  miles 
per  hour  multiplied  by  the  cosine  of  the  latitude.  At  30° 
north  and  south  an  observer  is  carried  900  miles  per  hour, 
at  45°  only  735  miles,  while  at  60°  this  is  reduced  to  500 
miles.  The  moon  moves  in  her  orbit  from  west  to  east  and 
her  shadow  in  consequence  traverses  the  earth  in  this  direc- 
tion also.  This  being  likewise  the  direction  that  an  observer 
is  carried  by  the  earth's  rotation,  the  moon's  shadow  path 
travels  with  respect  to  the  earth's  surface  the  difference  of 
the  two  speeds,  which  is  1060  miles  at  the  equator,  and 
1600  miles  at  latitude  60°. 

The  above  values  refer  only  to  the  condition  that  the 
eclipse  occurs  on  the  meridian  for  the  observer  in  question, 


46  ECLIPSES    OF    THE    SUN 

or  in  other  words,  at  noon.  When  the  eclipse  takes  place 
near  sunrise  or  sunset,  the  observer  being  turned  not  di- 
rectly towards  the  sun  and  moon,  the  projection  of  the 
moon's  orbital  velocity  along  the  earth's  surface  may  be 
very  large.  The  slowest  speed  that  the  moon's  shadow 
can  have  over  the  earth's  surface  is  1060  miles  per  hour, 
when  the  eclipse  occurs  at  noon  and  at  the  equator,  and  the 
speed  under  conditions  of  higher  latitudes  and  eclipse  at 
sunrise  or  sunset  may  be  as  great  as  4000  or  even  5000 
miles  per  hour.  With  the  conditions  causing  maximum 
width  of  shadow  of  163  miles,  an  eclipse  of  the  sun  may 
have  the  maximum  possible  duration  of  the  relatively  brief 
period  of  7  mins.  31  sees.  Even  a  six-minute  eclipse,  such 
as  the  Sumatra  eclipse  of  May  18,  1901  and  its  repetition 
eighteen  years  later  in  Brazil  and  Africa,  is  considered  by 
astronomers  unusually  long. 

The  discussion  above  regarding  the  conditions  of  a  solar 
eclipse  has  been  made  with  reference  to  the  umbra  of  the 
moon's  shadow.  An  observer  on  the  earth  situated  within 
the  confines  of  this  shadow  would  find  the  sun's  light  totally 
obscured.  Inside  the  penumbra  of  the  moon's  shadow,  the 
sun  is  not  entirely  covered  up  by  the  moon  and  a  partial 
solar  eclipse  will  result.  The  diameter  of  the  penumbra 
intercepted  by  the  earth  is  readily  determined.  Since  the 
angular  diameters  of  sun  and  moon  are  nearly  equal,  the 
diameter  of  the  penumbra  measured  at  right  angles  to  the 
line  joining  sun  and  moon  is  about  twice  the  diameter  of  the 
moon,  or  roughly  4400  miles.  Consequently,  for  a  distance 
of  2  200  miles  along  the  earth's  surface  on  either  side  of  the 
moon's  umbra  a  partial  eclipse  may  be  visible.  And  by  a 
process  of  reasoning  similar  to  that  followed  out  in  the  case 
of  total  eclipses,  it  is  readily  perceived  that  in  latitudes  away 
from  the  equator  and  when  the  eclipse  is  at  sunrise  or  sunset, 
the  distance  along  the  earth's  surface  when  a  partial  solar 
eclipse  is  visible  may  be  increased  to  3000  miles  from  the 
central  line  of  totality. 

Again,  since  the  sun  and  moon's  angular  diameters  are 
about  equal,  and  the  moon  moves  the  extent  of  her  diam- 
eter in  an  hour,  it  will  take  about  one  hour  from  the  first 


THE    PREDICTION    OF   ECLIPSES  47 

beginning  of  an  eclipse,  either  solar  or  lunar,  to  the  beginning 
of  totality,  or,  as  the  astronomer  expresses  it,  from  first  to 
second  contact.  Similarly  an  hour  will  elapse  between  third 
and  fourth  contacts,  or  between  the  ending  of  totality  and 
the  end  of  the  eclipse. 


FIG.  2 

For  a  partial  eclipse  of  the  sun  to  occur,  it  is  necessary  for 
the  moon  at  M  to  encroach  on  the  shadow-cone  ACDB  en- 
veloping sun  and  earth.  The  angle  between  centers  of  sun 
and  moon  under  these  conditions  is  readily  determined. 
The  angle  SEM  in  Fig.  2  is  the  sum  of  three  angles,  SEA, 
AEF  and  FEM.  SEA  and  FEM  are  the  angular  semi-diam- 
eters of  sun  and  moon  respectively,  while  the  angle  AEF  is 
equal  CFE—  CAR.  But  CAR  is  the  angle  subtended  by  the 
earth's  radius  at  the  distance  of  the  sun,  and  is  accordingly 
the  sun's  horizontal  parallax.  If  S  and  s  represent  the  angu- 
lar semi-diameter  of  sun  and  moon,  and  P  and  p  the  hori- 
zontal parallaxes  of  sun  and  moon  respectively,  then  SEM 
is  equal  to  S  +  s  —  P  +  p. 

For  the  solar  eclipse  to  be  total,  the  moon  must  be  en- 
tirely within  the  shadow  at  m,  and  for  these  conditions  the 
angle  SEm  is  found  to  be  S  —  s  —  P  +  p. 

For  an  eclipse  of  the  moon,  with  moon  at  N,  the  angle  be- 
tween the  moon  and  the  center  of  the  earth's  shadow,  the 
angle  NET  is  equal  NEH  +  HET  =  NEH  +  CHE  —  HTE. 
But  SEA  =  EAT  +  HTE,  hence  NET  =  s+p  —  S  +  P. 

Since  the  centers  of  the  sun  and  earth  are  always  in  the 
ecliptic,  the  angles  just  determined  give  the  amount  in 
angle  that  the  moon  may  be  distant  from  the  ecliptic  at  the 
time  of  new  or  full  moon  in  order  that  there  may  be  an 


48  ECLIPSES    OF    THE    SUN 

eclipse  of  sun  or  moon.  Angular  distances  measured  at 
right  angles  to  the  ecliptic  are  called  celestial  latitudes,  and 
hence  in  order  that  a  partial  eclipse  may  take  place,  the 
latitude  of  the  moon  must  be  less  than 


and 


p  +  s  +  (S — P)         for  a  solar  eclipse, 
p  +  s  —  (S — P)         for  a  lunar  eclipse. 


Since  S  has  a  mean  value  of  32',  while  P  is  never  greater 
than  9",  it  is  evident  that  a  solar  eclipse  can  take  place 
with  the  moon  at  a  greater  angular  distance  from  the  ecliptic 
than  is  possible  for  an  eclipse  of  the  moon.  This  is  patent 
by  referring  to  Fig.  2  and  noting  that  a  section  across  the 
shadow-cone  enveloping  sun  and  earth  is  greater  through 
the  moon  at  new  moon,  i.e.,  for  a  solar  eclipse/than  at  full 
moon,  or  for  a  lunar  eclipse. 

For  the  conditions  under  which  total  eclipses  may  occur, 
we  find  that  the  celestial  latitudes  of  the  moon  at  the  time 
of  conjunction  of  sun  and  moon  are  (by  changing  +  s  to  —  s 
in  the  above  formula) 

p  —  s  +  (S — P)         for  total  solar  eclipse, 
and  p  —  s  +  (S — P)         for  total  lunar  eclipse. 

On  account  of  the  varying  distances  of  sun  and  moon 
from  the  earth,  the  values  of  the  semi-diameters  and 
parallaxes  of  sun  and  moon  are  changing  proportionally. 
In  the  table  below  the  values  are  taken  from  the  American 
Ephemeris: 

Greatest  Least  Mean 

Semi-diameter  of  sun             =5         16'  18"                 15'  46"  is'5o".6 

Semi-diameter  of  moon           =   s         16'  46"                 14' 43"  15' 32". 6 

Horizontal  parallax  of  sun     =  P                8".Q                      8".y  8".8 

Horizontal  parallax  of  moon  =  p         61'  28"                 53'  55"  57'    2".7 

Inclination  of  moon's  orbit    =   i  5°  19'  4°  57'  5°    8'  43" 

An  eclipse,  either  of  sun  or  moon,  might  be  predicted  by 
finding  from  the  above  formulas  the  moon's  celestial  lati- 
tude. For  purposes  of  computation  it  is  more  convenient 
to  find  the  angular  distance  that  the  sun  or  moon  may  be 
from  the  moon's  node,  but  measured  in  the  plane  of  the 
ecliptic.  These  angles,  being  celestial  longitudes,  deter- 


THE    PREDICTION    OF    ECLIPSES  49 

mined  at  the  time  of  new  moon  give  what  is  known  as  the 
solar  ecliptic  limit,  and  at  full  moon  furnish  the  lunar  eclip- 
tic limit.  In  Fig.  3,  NS  is  the  ecliptic,  MN  the  moon's  path, 
N  the  moon's  ascending  node.  M  is  the  moon,  and  the  circle 
at  S,  for  a  solar  eclipse,  represents  a  section  of  the  shadow 
cone  at  M  (in  Fig.  2),  and,  for  a  lunar  eclipse,  a  section 
through  N  (in  Fig.  2),  The  angle  SNM  represents  the 
angle,  /,  of  inclination  of  the  moon's  orbit  to  the  plane  of 
the  ecliptic. 


M 


FIG.  3 

For  a  solar  eclipse,  the  angular  distance  SM  is  known 
from  above  as  equal  to  p  +  s  +  S — -P.  The  angle  i  is  known, 
and  if  the  angle  at  S  is  assumed  as  a  right  angle  the  tri- 
angle SNM  can  be  solved,  and  the  distance  SN  determined. 
An  approximate  method  is  all  that  is  required,  and  since 
the  mean  value  of  i  is  nearly  one-eleventh  of  a  radian,  hence 
SN,  the  ecliptic  limit  is  approximately  eleven  times  SM. 
But  as  the  value  of  SM  depends  on  the  semi-diameters 
and  parallaxes  of  sun  and  moon,  and  as  we  have  seen  these 
quantities  continually  are  varying,  the  value  of  the  ecliptic 
limit  must  also  vary  proportionally,  and  also  because  the 
angle  i  changes  likewise.  Hence  it  is  necessary  to  dis- 
tinguish between  the  maximum  and  minimum  values  of  the 
ecliptic  limit,  for  solar  and  lunar  eclipses  and  for  total  and 
partial  eclipses.  Using  the  appropriate  maximum  and  min- 


50  ECLIPSES    OF    THE    SUN 

imum  values,  it  is  easy  to  find  the  following  values  of 
eclipse  limits. 

Major         Minor 

Solar  eclipse 18°  31'  15°  21' 

Lunar  eclipse 12°  15'  9°  30' 

Total  solar  eclipse 11°  50'  9°  55' 

Total  lunar  eclipse 6°    o'  3°  45' 

As  a  simple  exercise  in  mathematical  astronomy  these 
quantities  might  be  used  for  predicting  eclipses.  All  of  the 
additional  information  is  readily  obtainable  from  one  of  the 
government  publications  like  the  American  Ephemeris  and 
Nautical  Almanac.  As  already  stated,  an  eclipse  of  the  sun 
can  take  place  only  at  new  moon.  For  this  instant  of  time 
look  up  in  the  Ephemeris  the  longitude  of  the  sun  and  that 
of  the  moon's  node.  The  difference  is  the  angular  distance 
of  the  sun  from  the  node.  If  this  longitude  is  greater  than 
the  major  ecliptic  limit  of  i8°3i',  an  eclipse  of  the  sun 
cannot  possibly  take  place;  if  the  quantity  is  less  than  the 
minor  limit  of  i5°2i',  an  eclipse  must  certainly  take  place. 
If  the  difference  in  longitudes  is  less  than  i8°3i',  but 
greater  than  15°  21',  it  is  impossible  to  state  whether  an 
eclipse  will  take  place  or  not,  and  recourse  must  then  be  had 
to  a  calculation  of  the  ecliptic  limit  using  the  particular 
values  for  the  semi-diameters,  parallaxes  and  inclination  for 
the  day  in  question  at  the  time  of  new  moon.  The  condi- 
tions for  a  total  eclipse  are  found  by  using  the  major  and 
minor  ecliptic  limits  of  n°So/  and  9° 5 5'.  Similarly,  for  an 
eclipse  of  the  moon,  which  occurs  only  at  full  moon,  by 
utilizing  the  values  of  the  lunar  ecliptic  limits,  it  is  possible 
to  predict  whether  an  eclipse  of  the  moon  will  happen  or  not. 

In  practice,  however,  in  predicting  the  number  of  eclipses 
to  occur  in  any  one  year,  the  eclipse  limits  defined  above 
are  not  used.  It  is  much  simpler  to  use  the  Saros  to  bring 
forward  the  data  of  the  Ephemeris  of  18  years  before.  All 
of  the  various  series  of  eclipses,  lunar  and  solar,  that  are  tak- 
ing place  at  the  present  time  are  known  to  the  astronomer. 
When  no  great  accuracy  is  needed  the  Saros  provides  a  sim- 
ple method.  The  information  so  obtained  should  be  checked 
by  the  magnificent  book  by  Oppolzer,  Canon  der  Finsternisse 


THE    PREDICTION    OF    ECLIPSES  51 

where  are  given  the  elements  of  all  of  the  eclipses  (8000 
solar  and  5200  lunar)  between  the  dates  1208  B.C.  and  2162 
A.D.  Various  lists  of  total  solar  eclipses  have  been  pub- 
lished. (See  for  instance.  Encyclopaedia  Britannica,  article 
on  Eclipses}. 

Referring  now  to  the  lengths  of  the  different  kinds  of 
months  on  p.  40,  we  recall  that  19  returns  of  the  sun 
to  the  moon's  node  are  equal  to  6585.7806  days,  while 
223  synodic  months  consume  the  slightly  different  amount 
°f  6585.3211  days.  If  therefore  a  new  moon  fell  exactly 
at  the  node,  then  after  18  years  n  days,  the  new  moon 
would  take  place  before  the  node  was  reached.  The 
difference  of  the  two  quantities  above  is  0.4595  days,  in 
which  time  the  sun  moves  28'.  Accordingly,  at  each  suc- 
ceeding Saros  the  new  moon  is  found  28'  farther  and  farther 
west  of  the  node.  It  is  now  possible  to  trace  the  total 
number  of  eclipses  and  the  progressive  changes  in  them  as 
they  pass  through  the  Saros.  According  to  the  value  of 
the  ecliptic  limit  already  found,  we  know  that  if  the  new 
moon  happens  within  18°  of  the  node,  an  eclipse  of  the  sun 
may  take  place.  If  the  node  is  the  ascending  one,  the  con- 
ditions will  be  as  represented  in  Figs.  3  and  2,  the  moon 
being  north  of  the  shadow-cone.  An  eclipse  of  the  sun 
will  accordingly  be  visible  in  high  northern  latitudes  on  the 
earth.  After  18  years  n  days  the  conditions  will  be  nearly 
identical,  but  the  new  moon  will  take  place  28'  nearer  the 
node,  and  as  a  result,  the  eclipse  will  be  visible  on  the  earth 
a  little  farther  south  of  the  preceding  position.  With  each 
succeeding  return,  the  new  moon  moves  nearer  and  nearer 
the  node  and  the  eclipse  track  shifts  farther  and  farther 
south  on  the  earth.  When  the  moon  is  within  about  11° 
of  the  node  the  solar  eclipse  becomes  central,  and  the  eclipse 
may  be  total  or  annular.  As  before,  the  central  eclipse 
track  will  first  touch  only  high  northern  latitudes,  each 
succeeding  eclipse  moving  farther  and  farther  to  the  south. 
Total  or  annular  eclipses  will  now  take  place  at  each 
return  of  the  Saros  until  the  new  moon  takes  place 
11°  west  of  the  node,  when  the  central  eclipse  track 
passes  off  the  earth  at  its  south  pole,  and  a  series  of  partial 


52  ECLIPSES    OF    THE  SUN 

eclipses  ensues  until  the  moon  is  18°  west  of  the  node,  when 
these  particular  eclipses  cease.  In  such  a  series  there  are 
from  68  to  75  solar  eclipses,  depending  on  conditions,  extend- 
ing over  some  1200  years.  In  each  series  there  are  approxi- 
mately 25  partial  eclipses  and  45  central  eclipses.  The 
numbers  vary  for  different  series  of  eclipses.  Of  the  central 
eclipses,  total  eclipses  follow  total  eclipses  with  about  the 
same  duration  of  totality,  and  annular  eclipses  follow  an- 
nular. If  the  eclipses  had  taken  place  at  the  descending 
node  of  the  moon's  orbit  instead  of  at  the  ascending  node, 
eclipses  would  have  come  on  the  earth  at  its  south  pole  and 
gradually  moved  north,  going  off  the  earth  at  the  north  pole, 
as  shown  in  the  illustration  facing  this  page. 

On  account  of  the  ecliptic  limits  for  lunar  eclipses  being 
smaller  in  value  than  for  solar  eclipses,  there  will  be  fewer 
repetitions  in  a  lunar  series,  there  being  48  or  49  altogether. 
Of  this  number  there  will  be  22  or  23  total,  with  13  or  14 
partial  eclipses,  both  before  and  after  the  total  eclipses. 
The  interval  for  a  series  of  lunar  eclipses  consumes  about 
870  years. 

There  is  one  other  very  important  relation  brought  out 
by  the  data  on  page  41.  It  is  there  found  that  239  returns 
of  the  moon  to  the  line  of  apsides,  or  to  perigee,  amount  as 
a  total  to  6585.5374  days.  For  predicting  the  nature  of  the 
eclipse  that  will  be  found  at  the  next  return  this  quantity 
is  almost  as  valuable  as  the  Saros  itself.  At  the  end  of  223 
lunations  the  moon  not  only  returns  very  closely  to  its  origi- 
nal position  with  respect  to  the  sun  and  the  node,  but  also 
with  respect  to  the  line  of  apsides,  with  the  necessary  result 
that  the  distance  from  earth  to  moon  is  very  closely  re- 
peated. This  fact  in  turn  brings  two  consequences;  firstly 
that  the  duration  of  the  eclipse,  or  at  least  that  part  of  it 
which  depends  on  the  moon's  distance,  is  altered  but  little; 
but  secondly,  that  the  perturbations  of  the  moon's  orbit 
which  otherwise  might  have  displaced  by  several  hours  the 
time  of  the  eclipse  will  now  be  repeated  almost  as  they  were 
before,  and  with  no  consequent  relative  effect  on  the  time 
of  the  eclipse. 


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Oj-i-'UUT  TOWEK  TELESCOPE  OF  THE  SWARTHMORE  EXPEDITION  INSTALLED  IN 
SUMATRA  FOR  THE  1929  ECLIPSE 


CHAPTER  IV 

THE  VERIFICATION  OF  ECLIPSES 

IN  THE  last  chapter  it  was  shown  that  the  times  of 
eclipses  can  be  foretold  with  moderate  accuracy  by 
means  of  the  Saros.  Equally  important  with  the  actual 
prediction  of  the  times  is  the  fact  that  the  circumstances  at- 
tending the  eclipses  following  each  other  in  the  Saros  will  be 
very  closely  repeated.  A  large  partial  eclipse  of  the  sun  will 
be  followed  by  a  large  partial  eclipse  of  the  sun,  an  annular 
eclipse  by  an  annular  eclipse,  a  total  eclipse  of  short  dura,- 
tion  by  a  similar  short  eclipse.  And  so  also  with  lunar 
eclipses.  There  remains  still  to  explain  the  signification  of 
the  decimal  portion  of  the  time  of  the  Saros,  6585.3211  days. 
If  the  sun  had  been  on  the  meridian  at  the  middle  of  the 
eclipse,  the  eclipse  therefore  occurring  at  noon,  the  next 
following  eclipse  will  be  repeated  at  a  place  0.3211  of  a 
revolution  of  the  earth,  or  in  other  words  yh.  42m.  of  longi- 
tude farther  west.  After  the  return  of  three  Saroses,  or  54 
years,  the  eclipse  tracks  will  have  gone  almost  around  the 
world  and  will  have  returned  again  to  nearly  the  same 
longitude.  If  the  eclipses  belong  to  a  series  that  is  taking 
place  at  the  moon's  ascending  node,  the  later  eclipse  track 
will  be  found  farther  south  than  the  earlier  one,  and  if  at 
the  descending  node,  farther  north  than  the  eclipse  fifty- 
four  years  earlier. 

Those  who  wish  to  amuse  themselves  by  playing  with 
figures  may  find  other  remarkable  coincidences  by  experi- 
menting with  the  lengths  of  the  various  years  and  months 
on  p.  41.  Newcomb  has  found  a  very  interesting  period 
at  the  end  of  358  lunations. 

358  synodic  months  =   10571.95  days 
30.5  eclipse  years       =   10571.91  days 

53 


54  ECLIPSES    OF    THE    SUN 

This  period  amounts  to  29  Julian  years,  less  20.3  days.  But 
358  lunations  are  equal  in  length  to  383.673  anomalistic 
months,  and  hence,  as  explained  in  the  preceding  chapter, 
the  eclipses  which  follow  each  other  with  this  period  will 
differ  greatly  in  their  characteristics,  since  the  conjunctions 
between  sun  and  moon  take  place  at  different  positions  with 
respect  to  perigee.  Also,  such  eclipses  must  follow  each 
other  alternately  at  ascending  and  descending  nodes.  Three 
such  periods,  however,  will  equal  1169.019  anomalistic 
months,  and  the  time  of  conjunction  has  accordingly  moved 
very  close  to  perigee  (if  it  had  previously  been  found  there) 
and  a  total  eclipse  will  thus  be  followed  by  a  total  eclipse. 
Three  of  these  periods  equal  87  years  less  61  days,  and  18 
periods  equal  521  years,  actually  within  a  day  or  two.  Thus 
with  the  Saros  of  18  years,  and  with  the  addition  of  the 
29-year,  the  87-year  and  the  52i-year  periods,  the  astrono- 
mer is  enabled  to  engage  in  predicting  at  long  range.  Take 
for  instance  the  first  eclipse  verified  with  certainty,  the 
Nineveh  eclipse  of  June  15,  763  B.C.  (763  B.C.  may  be  ex- 
pressed also  as  the  year  —762).  Applying  the  52i-year 
period  we  have  the  years  242  B.C.  and  A.D.  280,  801,  1322 
and  1843.  These  eclipses  each  fell  on  June  15,  0.  S.,  or  by 
the  Julian  calendar.  The  date  of  June  15,  1843,  O.  S.,  is  the 
same  as  June  27,  1843,  by  the  Gregorian  or  present-day 
calendar.  By  means  of  the  29-year  period  we  obtain  the 
eclipses  of  June  6,  1872,  and  May  18,  1901.  (The  eclipses 
of  1843  and  *872  were  not  total  while  that  of  1901  was  the 
six-minute  total  eclipse  observed  in  Sumatra.) 

Facing  page  56  will  be  seen  a  map  of  the  tracks  of  all  of 
the  eclipses,  total  and  annular,  taking  place  between  the 
years  1919  and  1940.  The  map  is  from  Oppolzer's  Canon 
der  Finsternisse. 

In  the  following  table  are  given  one  hundred  years  of 
total  eclipses  of  the  sun.  The  article  on  "  Eclipses  "  in  Ency- 
clopaedia Britannica,  i4th  edition,  does  not  contain  the 
eclipses  of  1944,  1945, 1950,  and  1968.  As  shown  in  the  dia- 
gram facing  page  52,  the  last  two  are  unimportant  eclipses 
which  occur  in  the  north  polar  regions. 


THE  VERIFICATION  OF  ECLIPSES 


55 


Date 

Series 

Node 

Duration 
in  minutes 

Where  visible 

1875,  April  6 

i 

A 

4-7 

Indian  Ocean,  Siam,  Pacific. 

1876,  Sept.  17 

2 

D 

1.8 

Pacific  Ocean. 

1878,  July  29 

3 

D 

3.2 

Canada  and  the  United  States. 

1880,  Jan.  ii 

4 

A 

2  .  I 

Pacific  Ocean,  California. 

1882,  May  17 

$ 

D 

1.8 

Egypt,  Central  Asia,  China. 

1883,  May  6 

6 

D 

6.0 

Pacific  Ocean,  Caroline  Islands. 

1886,  Aug.  29 

7 

A 

6.6 

South  America,  Central  Africa. 

1887,  Aug.  19 

8 

A 

3-8 

Northern  Europe,  Siberia,  Japan. 

1889,  Jan.  i 

9 

1) 

2.  2 

California,  Oregon,  Canada. 

1889,  Dec.  22 

10 

D 

4.2 

South  America  and  Central  Africa. 

1893,  April  1  6 

i 

A 

4.8 

Venezuela  to  West  Africa. 

1894,  Sept.  29 

2 

D 

1.8 

East  Africa,  Indian  Ocean. 

1896,  Aug.  Q 

3 

D 

2.7 

North  Europe,  Siberia,  Japan. 

1898,  Jan.  22 

4 

A 

2-3 

East  Africa,  India,  China. 

1900,  May  28 

5 

D 

2  .  I 

United  States,  Spain,  North  Africa. 

1901,  May  18 

6 

D 

6.5 

Sumatra,  Borneo. 

1904,  Sept.  9 

7 

A 

6.4 

Pacific  Ocean. 

1905,  Aug.  30 

8 

A 

3-8 

Canada,  Spain,  North  Africa. 

1907,  Jan.  14 

9 

D 

2-3 

Russia,  Central  Asia. 

1908,  Jan.  3 

10 

D 

4.2 

Pacific  Ocean. 

1911,  April  28 

i 

A 

5.° 

Australia,  Polynesia. 

1912,  Oct.  10 

2 

D 

1.8 

Colombia,  Ecuador,  Brazil. 

1914,  Aug.  21 

3 

D 

2.1 

Scandinavia,  Russia,  Asia  Minor. 

1916,  Feb.  3 

4 

A 

2-5 

Pacific  Ocean,  Venezuela. 

1918,  June  8 

<> 

D 

2.4 

The  United  States. 

1919,  May  29 

6 

D 

6.9 

Peru,  Brazil,  Central  Africa. 

1922,  Sept.  21 

7 

A 

6.1 

East  Africa,  Australia. 

1923,  Sept.  10 

8 

A 

3-6 

California,  Mexico,  Central  America. 

1925,  Jan.  24 

9 

D 

2    4 

Northeastern  United  States. 

1926,  Jan.  14 

10 

D 

4.2 

East  Africa,  Sumatra,  Philippines. 

1927,  June  29 

IT 

A 

0.7 

England,  Scandinavia. 

1929,  May  9 

i 

A 

5-1 

Sumatra,  Siam,  Philippines. 

1930,  Oct.  21 

2 

D 

1.9 

Pacific  Ocean,  Patagonia. 

1932,  Aug.  31 

3 

D 

i-5 

Canada,  New  England. 

1934,  Feb.  14 

4 

A 

2.7 

Borneo,  Celebes,  Caroline  Islands. 

1936,  June  19 

5 

D 

2.5 

Greece  to  Central  Asia  and  Japan. 

1937,  June  8 

6 

D 

7-i 

Pacific  Ocean,  Peru. 

1940,  Oct.  i 

7 

A 

5-7 

Colombia,  Brazil,  South  Africa. 

1941,  Sept.  21 

8 

A 

3-3 

Central  Asia,  China,  Pacific  Ocean. 

1943,  Feb.  4 

9 

D 

2-5 

China,  Pacific  Ocean,  Alaska. 

1944,  Jan.  25 

10 

D 

4-i 

South  America,  West  Africa. 

1945,  July  9 

ii 

A 

i  .1 

Canada,    Greenland,    Scandinavia, 
Russia. 

1947,  May  20 

i 

A 

5-2 

South  America,  Africa. 

1948,  Nov.  i 

2 

D 

1.9 

Central  Africa,  Madagascar. 

1950,  Sept.  12 

3 

D 

0.9 

Northeastern  Siberia. 

1952,  Feb.  25 

4 

A 

3-° 

Africa,  Persia,  Central  Asia. 

1954,  June  30 

5 

D 

2-5 

Canada,  Scandinavia,  Russia. 

56 


ECLIPSES  OF  THE  SUN 


Date 

Series 

Node 

Duration 
in  minutes 

Where  visible 

I9S5»  June  20 

6 

D 

7.2 

Ceylon,  Siam,  Philippines. 

1958,  Oct.  12 

7 

A 

5-2 

Chile,  Argentina. 

1959,  Oct.  2 

8 

A 

3-° 

Canaries,  Central  Africa. 

1961,  Feb.  15 

9 

D 

2.6 

France,  Italy,  Austria,  Siberia. 

1962,  Feb.  5 

10 

D 

41 

New  Guinea. 

1963,  July  20 

ii 

A 

i-5 

Alaska,  Canada,  Maine. 

1965,  May  30 

i 

A 

5-3 

Pacific  Ocean. 

1966,  Nov.  12 

2 

D 

1.9 

Bolivia,  Argentina,  Brazil. 

1968,  Sept.  22 

3 

D 

0-5 

Northern  Siberia. 

1970,  Mar.  7 

4 

A 

3-3 

Mexico,  Florida,  Georgia. 

1972,  July  10 

5 

D 

2.7 

Northeast  Asia,  Northeast  Amer- 

• 

ica  and  Atlantic  Ocean. 

1973,  June  30 

6 

D 

7.2 

South  America,  Africa. 

1974,  June  20 

12 

D 

5-3 

Southwest    Australia    and    Indian 

Ocean. 

From  the  table  it  will  be  seen  that  the  total  eclipses  which 
have  been  or  will  be  observed  in  the  United  States  are:  1878, 
July  29;  1880,  January  n;  1889,  January  i;  1900,  May  28; 
1918,  June  8;  1923,  September  10;  1925,  January  24;  1932, 
August  31 ;  1963,  July  20;  1970,  March  7.  So  many  eclipses 
in  the  past,  so  few  in  the  coming  years!  In  the  future,  the 
next  eclipses  to  visit  the  United  States  will  be  those  of  1945 
and  1954,  both  of  which  begin  at  sunrise  about  on  the  interna- 
tional boundary  and  their  tracks  will  go  northwards  over  inac- 
cessible spots  in  Canada.  To  observe  either  of  these  eclipses 
it  will  probably  be  necessary  to  travel  to  Scandinavia.  The 
next  eclipse  to  be  observed  scientifically  in  the  United  States 
is  that  of  July  20,  1963,  the  path  crossing  Maine  not  very  far 
northeast  of  the  1932  eclipse.  The  eclipse  of  March  7, 1970, 
will  be  visible  in  Florida  and  along  the  coast  of  Georgia.  The 
next  American  eclipse  after  that  will  be  on  February  26, 1979, 
visible  in  northwest  United  States  and  in  Canada  across  Hud- 
son Bay.  What  hardy  astronomer  will  be  willing  to  brave 
the  probabilities  of  a  blizzard  and  40°  F.  below  zero  in  order 
to  add  to  scientific  knowledge  by  observing  this  total  eclipse? 

In  the  immediate  future,  the  next  total  eclipse  to  be  ob- 
served will  be  on  June  19,  1936,  with  a  duration  of  two  and  a 
half  minutes.  At  sunrise  the  total  eclipse  begins  near  Greece, 
the  path  of  totality  later  passing  through  Siberia  and  Japan. 


TRACKS  OF  ECLIPSES  BETWEEN  THE  YEARS  1919  AND  1940 
From  Oppolzer's  Canon  der  Finsternisse. 


DESOLATION  ON  NIUAFOOU  ISLAND  AFTER  THE  ERUPTION  IN  1929 


HUGE  CAMERAS  INSTALLED  FOR  THE  1930  ECLIPSE 


THE  VERIFICATION  OF  ECLIPSES  57 

The  most  favored  sites  for  observation  will  be  at  the  cities  of 
Omsk  and  Tomsk  on  the  Trans-Siberian  Railroad.  The  as- 
tronomers from  the  United  States  will  probably  locate  in 
Japan.  The  following  eclipse  is  June  8,  1937,  with  a  du- 
ration of  totality  of  more  than  seven  minutes.  The  track 
passes  across  the  wide  expanse  of  waters  of  the  Pacific  Ocean 
and  touches  land  only  near  sunset  in  Peru.  With  the  sun  at 
a  very  low  altitude  it  appears  probable  that  no  important  sci- 
entific observations  will  be  possible.  The  next  eclipse,  that 
of  October  i,  1940,  will  probably  be  extensively  observed 
in  South  Africa.  Evidently  the  coming  generation  of  Ameri- 
can astronomers  will  be  forced  to  make  long  trips  away  from 
home  if  they  expect  to  continue  the  important  discoveries 
made  at  eclipses.  At  the  present  time  many  eclipse  problems, 
discussed  in  the  final  chapter,  still  await  solution.  It  is  con- 
fidently expected,  however,  that  the  coming  generation  will 
find  methods  of  investigating  chromosphere  and  corona  with 
such  success  that  observations  at  eclipses  will  no  longer  be 
necessary.  When  science  shall  have  progressed  to  this  ex- 
tent, future  astronomers  will  be  able  to  point  to  the  long 
trips  taken  by  their  forefathers,  sometimes  half  way  round 
the  globe,  in  order  to  observe  a  total  eclipse  for  a  few  brief 
minutes  of  time.  In  the  twenty-first  century  the  great  god 
of  Efficiency  may  be  worshipped  even  more  profoundly  than 
is  the  case  in  the  early  part  of  the  twentieth  century.  Under 
such  circumstances  a  total  eclipse  will  still  be  viewed  with 
interest  as  a  fascinating  phenomenon,  which,  however,  will 
afford  ample  evidence  of  the  crude  methods  of  investigation 
of  the  astronomers  a  century  before. 

If  so  few  eclipses  are  visible  in  such  a  large  country  as  the 
United  States,  how  about  those  in  a  relatively  small  territory 
like  the  British  Isles?  The  eclipse  of  June  29,  1927  was  en- 
thusiastically observed  in  the  northern  portion  of  England 
in  spite  of  heavy  clouds;  that  of  August  n,  1999,  grazes  the 
south  of  Ireland  and  Land's  End,  and  on  August  12,  2026, 
the  eclipse  track  may  cross  the  southwestern  tip  of  Ireland 
but  will  not  be  seen  in  Great  Britain. 

Since  the  sixth  century  the  following  total  eclipses  have 
touched  parts  of  the  British  Isles: 


58  ECLIPSES  OF  THE  SUN 

594  July  23  1185  May  i 

603  Aug.  12  1330  July  1 6 

639  Sept.  3  1424  June  26 

664  May  i  1433  June  17 

878  Oct.  29  1598  Mar.  6 

885  June  15  1652  April  8 

1023  Jan.  24  1715  May  2 

1133  Aug.  i  1724  May  22 

1140  Mar.  20  1927  June  29 

Eclipses  have  included  London  twice,  Dublin  twice,  and 
Edinburgh  five  times. 

On  the  other  hand,  some  parts  of  the  globe  are  visited  by 
eclipses  in  rapid  succession.  Spain  witnessed  the  eclipses 
of  1842,  1860,  1870,  1900  and  1905.  In  the  vicinity  of  the 
East  Indies  the  eclipses  of  May  18,  1901,  January  14,  1926, 
May  9,  1929  and  February  14,  1934  have  been  observed, 
while  the  track  of  the  eclipse  of  September  21,  1922,  lay  but 
a  few  hundred  miles  south. 

Assuming  that  the  moon's  shadow  path  intercepted  by 
the  earth  averages  100  miles  in  width,  it  is  easy  to  find  that  on 
the  average  a  total  eclipse  of  the  sun  will  be  visible  in  any  one 
locality  once  every  360  years.  A  stay-at-home  astronomer 
might  have  little  opportunity  during  his  lifetime  to  witness 
many  such  eclipses ! 

With  a  knowledge  of  the  ecliptic  limits  found  on  page  50, 
it  is  possible  to  find  the  number  of  eclipses  that  will  occur 
in  any  given  time,  say  a  year.  If  at  the  time  of  the  new  moon, 
the  moon  (and  also  the  sun)  is  more  than  18°  31'  from  the 
moon's  node  there  cannot  possibly  be  an  eclipse  of  the  sun. 
Since  the  angle  of  18°  31'  may  be  east  or  west  of  the  node, 
there  is  thus  a  zone  of  37°,  within  which  an  eclipse  of  the 
sun  may  take  place  provided  that  a  new  moon  also  occurs 
somewhere  within  the  zone.  The  minor  ecliptic  limit  for  a 
solar  eclipse  is  15°  21',  and  similarly  if  new  moon  is  found 
anywhere  in  the  zone  30°  42'  in  length  an  eclipse  of  the  sun 
must  happen,  there  being  no  possibility  of  the  moon  slipping 
by  without  intercepting  the  light  from  the  sun  as  it  comes  in 
the  direction  towards  the  earth.  For  an  eclipse  of  the  moon, 
the  ecliptic  limits  are  smaller.  It  is  only  when  full  moon 
comes  within  9°  30"  of  the  node  that  a  lunar  eclipse  is  certain 
to  take  place,  though  an  eclipse  may  possibly  happen  if  the 


THE  VERIFICATION  OF  ECLIPSES  59 

limit  is  as  great  as  12°  15'.  In  a  synodic  month  the  earth 
moves  along  the  ecliptic  to  an  average  extent  of  29°  6'.  But 
the  moon's  node  is  moving  in  the  opposite  direction  by  an 
amount  of  i°  31'  each  month.  Relative  to  the  node,  there- 
fore, new  and  full  moon  are  found  farther  east  each  synodic 
month  by  the  sum  of  29°  6'  and  i°  31',  or  30°  37',  and  it  is 
this  motion  relative  to  the  node  on  which  the  circumstances 
of  an  eclipse  depend.  By  an  analogy  one  can  see  how  easily 
the  question  of  eclipses  is  thought  out.  Suppose  a  man  walk- 
ing around  a  circular  track  of  any  diameter  in  which  there  are 
two  mud  holes  four  feet  in  width  diametrically  opposite  each 
other.  If  he  is  taking  a  constant  pace  of  three  feet,  it  will  be 
impossible  for  him  to  walk  around  the  track  without  stepping 
at  least  once  into  each  mud  hole  during  each  circuit.  If 
when  he  comes  to  the  muddy  spot  he  happens  to  plant  his 
foot  near  the  middle  he  will  have  only  one  wet  foot,  but  if 
the  first  foot  gets  in  the  hole  near  the  edge,  the  second  foot 
may  be  planted  in  the  mud  near  the  other  side  of  the  puddle. 
If  the  mud  hole  were  only  two  feet  in  width,  his  standard 
pace  of  three  feet  might  carry  him  across  it  without  either 
foot  getting  into  the  mud.  And  so  it  is  with  eclipses  of  sun 
and  moon.  The  moon's  step  is  30°  37'  in  angular  length, 
the  danger  zone  for  a  solar  eclipse  is  30°  42 ',  or  a  quantity 
which  is  larger  than  the  moon's  monthly  motion  with  re- 
spect to  the  node.  There  must,  therefore,  be  at  least  one 
solar  eclipse  at  each  nodal  point,  or  two  such  eclipses  with- 
out fail  during  the  course  of  each  and  every  year.  On  the 
other  hand,  the  danger  zone  for  lunar  eclipses  being  much 
smaller,  it  is  possible  that  the  full  moon  may  take  place  so 
far  from  the  node  each  time  that  there  will  not  be  a  single 
lunar  eclipse  during  the  year.  There  can,  moreover,  be  only 
one  eclipse  of  the  moon  at  each  node,  but  never  two. 
Eclipses  do  not  occur  in  the  same  months  each  year,  but 
follow  each  other  at  the  beginning  and  end  of  the  "  eclipse 
year  "  of  346.6  days.  This  being  18  days  less  than  the  calen- 
dar year  it  is  possible  for  there  to  be  three  eclipses  of  the 
moon  within  the  calendar  year,  if  the  node  is  passed  early  in 
January,  or  late  in  December  (which  comes  to  the  same 
thing).  Two  solar  eclipses  may  possibly  take  place  at  each 


60  ECLIPSES  OF  THE  SUN 

node,  and  a  fifth  one  within  the  calendar  year.  If  the  new 
moon  take  place  near  the  node  bringing  an  eclipse  of  the  sun, 
the  preceding  or  following  full  moon  may  be  too  far  from  the 
node  to  cause  an  eclipse  of  the  moon.  Under  these  conditions 
there  will  be  only  two  eclipses  during  the  year,  each  of  the  sun, 
and  as  these  occur  near  the  node,  they  will  be  central  eclipses, 
namely,  either  total  or  annular.  This  frequently  happens,  as, 
for  instance,  in  the  years  1922, 1926,  1929,  1933,  1940,  1944, 
1951  and  1962.  If  there  are  two  solar  eclipses  at  each  node, 
a  full  moon  must  take  place  near  the  node  and  a  total  eclipse 
of  the  moon  will  result.  There  is  thus  the  possibility  of  two 
partial  solar  and  one  total  lunar  eclipse  at  each  node,  or  a 
total  of  two  lunar  and  four  solar  eclipses  during  the  year. 
If  the  node  is  passed  about  the  middle  of  January,  there  may 
be  a  fifth  solar  eclipse  within  the  calendar  year,  making  a 
maximum  of  seven  eclipses  in  a  single  year.  There  may 
also  be  three  eclipses  of  the  moon  and  four  eclipses  of  the 
sun  within  the  year,  an  event  which  will  happen  next  in  1982, 
the  three  lunar  eclipses  each  being  total,  and  the  four  solar, 
partial.  In  1 93  5  there  will  be  two  lunar  and  five  solar  eclipses. 
This  combination  occurs  but  seldom,  and  will  not  happen 
again  until  the  year  2485.  The  maximum  number  of  eclipses 
falling  within  a  calendar  year  is  thus  seven  and  the  minimum 
two. 

According  to  Oppolzer,  there  are  237.5  solar  eclipses  on 
the  average  in  a  century.  Of  these  83.8  are  partial,  77.3 
annular,  10.5  annular  and  total  (the  vertex  of  the  moon's 
shadow  just  reaching  the  earth)  and  65.9  total  eclipses. 
Twenty-seven  percent  of  all  solar  eclipses  are  total.  On  the 
average,  therefore,  two  total  eclipses  of  the  sun  are  visible 
every  three  years,  but  fully  half  of  these  are  inaccessible  for 
the  reason  that  the  eclipse  tracks  are  in  high  northern  or 
southern  latitudes,  or  pass  entirely  across  the  oceans,  or  lie 
in  localities  where  the  probability  of  good  weather  is  un- 
promising. Consequently,  on  the  average,  about  once  in 
every  three  years  a  total  eclipse  somewhere  on  the  earth  is 
available  for  observation.  As  the  duration  of  totality  aver- 
ages less  than  three  minutes,  we  thus  see  that  an  eclipse 
observer  may  have  an  average  of  a  minute  per  year  for 


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THE  VERIFICATION  OF  ECLIPSES  61 

scientific  work,  provided  he  has  sufficient  financial  backing 
to  permit  him  to  travel  long  distances  for  every  available 
eclipse.  The  author  has  had  the  great  good  fortune  of  wit- 
nessing nine  total  eclipses  in  1900,  1901,  1905,  1918,  1923, 
1925, 192  7, 1930  (Oct.  21)  and  1932.  In  1936  he  hopes  to  be 
able  to  observe  his  tenth  eclipse  when  he  will  have  accom- 
plished the  rare  feat  of  observing  three  total  eclipses  in  the 
same  series  separated  by  the  Saros  interval  of  18  years  n 
days. 

The  Lick  Observatory  has  set  up  an  enviable  reputation 
for  itself  through  the  observation  of  fifteen  total  eclipses  in 
the  following  years:  1889  (Jan.  i  and  Dec.  22),  1893,  1896, 
1898,  1900,  1901, 1905  (three  parties  in  Labrador,  Spain  and 
Egypt)?  i9°8,  1914?  1918,  1922,  1923,  1930  (April  28)  and 
1932.  In  the  past  there  have  been  but  three  complete  failures 
through  clouds,  in  1896,  1914  and  1923.  Excellent  observa- 
tions have  been  secured  by  the  British  Joint  Permanent 
Eclipse  Committee  under  the  auspices  of  the  Royal  Society 
and  the  Royal  Astronomical  Society.  Through  the  coopera- 
tion of  astronomers  with  eclipse  experience  working  under 
the  auspices  of  the  Commission  on  Eclipses  of  the  Inter- 
national Astronomical  Union,  problems  are  now  carefully 
studied  before  each  eclipse  to  the  end  that  there  may  be  little 
wasted  effort. 

Taking  into  account  the  earth  as  a  whole,  it  is  evident  that 
there  are  more  solar  eclipses  than  lunar,  in  the  approximate 
ratio  of  four  to  three.  But  for  eclipses  visible  from  any  one 
locality,  London  or  New  York  or  Timbuctoo,  the  question  is 
an  entirely  different  one.  A  partial  solar  eclipse  at  best  may 
be  seen  from  a  limited  portion  of  the  globe,  while  a  total 
eclipse  is  observable  only  in  a  very  restricted  path.  An 
eclipse  of  the  moon,  being  caused  by  its  passage  into  the 
shadow  of  the  earth,  must  be  visible  to  every  locality  on  the 
earth  where  the  moon  itself  is  visible.  At  any  one  instant 
of  time  the  moon  is  above  the  horizon  to  half  of  the  world, 
and  since  the  eclipse  of  the  moon,  from  beginning  to  ending 
of  the.  partial  eclipse,  lasts  for  some  hours  it  will  be  plainly 
visible  to  over  half  the  globe,  unless  clouds  interfere.  There 
are  probably  very  few  people  living  who  love  Nature,  who 


62  ECLIPSES    OF    THE    SUN 

have  not  witnessed  at  least  one  total  eclipse  of  the  moon,  but 
probably  not  more  than  one  in  a  thousand  of  the  world's 
inhabitants  has  caught  a  glimpse  of  the  matchless  glory  of 
the  corona,  seen  only  during  the  fleeting  moments  of  a 
total  eclipse  of  the  sun. 

CALCULATION  OF  AN  ECLIPSE 

The  seemingly  uncanny  and  almost  miraculous  power  of 
the  astronomer  to  predict  the  coming  of  an  eclipse  hun- 
dreds of  years  in  advance  has  always  possessed  a  powerful 
fascination  for  the  uninitiated.  By  means  of  the  Saros 
and  of  the  methods  outlined  in  the  foregoing  chapter,  it  is 
readily  possible  to  foretell  the  happening  of  an  eclipse  of  the 
sun  or  moon  and  the  general  circumstances  surrounding  these 
events,  provided  no  very  great  accuracy  of  time  or  place  is 
required.  To  know  the  exact  times  for  any  given  locality 
more  accurate  methods  must  be  used.  The  problem  of 
eclipse  calculations  resolves  itself  into  three  stages.  First, 
the  accuracy  of  the  whole  problem  depends  mainly  on  the 
completeness  of  our  knowledge  regarding  the  motions  of 
the  moon  furnished  by  the  lunar  theory,  and  also  upon  the 
motion  of  the  earth  about  the  sun  which  gives  the  apparent 
motion  of  the  sun  about  the  earth,  the  latter  theory  being 
comparatively  simple.  From  the  motions  of  sun  and  moon 
thus  determined,  it  is  necessary  to  compute  their  positions  at 
equidistant  intervals,  and  as  seen  from  a  standard  place, 
namely,  the  center  of  the  earth.  Second,  from;  these  posi- 
tions it  is  necessary  to  compute  certain  "  elements  "  on 
which  eclipses  depend;  and  third,  for  a  given  latitude  and 
longitude  of  any  place  on  the  earth's  surface,  to  calculate 
the  times,  etc.,  of  the  eclipse.  The  first  part  of  the  problem 
is  taken  care  of  by  the  various  governmental  publications 
which  appear  some  three  years  in  advance.  By  this  means 
it  is  readily  possible  to  find  the  exact  positions  of  sun 
and  moon  at  any  given  instant  of  time.  Elements  of  eclipses 
needed  for  the  second  step  of  the  process  have  been  calcu- 
lated, from  the  earliest  historic  times  up  to  the  twenty- 
second  century,  by  Oppolzer  and  Simon  Newcomb.  The 


THE    VERIFICATION    OF   ECLIPSES  63 

third  step  in  computing  the  details  of  the  eclipses  for  a  given 
year  is  now  done  by  the  American  Ephemeris  alone,  but  the 
results  are  published  by  the  other  nautical  almanacs,  thus 
eliminating  duplication  of  efforts.  For  the  1932  eclipse 
Oppolzer's  chart  was  in  error  by  200  miles  in  giving  the 
eclipse  path;  his  elements  however  furnished  the  position 
of  the  central  line  within  an  error  of  10  or  15  miles  and  the 
times  to  an  accuracy  of  about  one  minute.  The  American 
Ephemeris  aims  at  an  accuracy  of  one-quarter  mile  in  posi- 
tion and  two  seconds  for  the  times  of  contacts. 

The  calculation  of  an  eclipse  of  the  moon  is  one  of  great 
simplicity.  The  time  at  which  the  moon  passes  into  the 
shadow  cast  by  the  earth  is  the  same  absolute  instant  of 
time  no  matter  where  the  observer  is  on  the  earth.  Even 
the  inhabitant  of  Mars  (who  may  or  who  may  not  exist), 
who  observes  the  earth  for  signs  of  life  with  the  gigantic 
telescope  which  his  supposed  advanced  civilization  must 
have  furnished  him,  would  see  the  eclipse  of  the  moon  at 
identically  the  same  absolute  second  of  time  as  that  noted 
by  the  Astronomer  Royal  at  Greenwich.  The  local  time  of 
the  eclipse  recorded  by  the  Martian  clock  would  not  be 
Greenwich  mean  time,  —  but  that  is  not  part  of  the  prob- 
lem. Eclipses  of  Jupiter's  satellites,  caused  by  the  passing 
of  these  moons  into  the  shadows  cast  by  the  planet  are  seen 
at  the  same  instant  of  time  by  all  observers  on  the  earth, 
provided  of  course  that  the  terrestrial  observers  are  on  the 
side  of  the  earth  turned  towards  Jupiter.  If  the  various 
observers  take  the  value  of  the  Greenwich  mean  time  of 
such  an  eclipse  (which  can  be  obtained  from  the  nautical 
almanacs)  and  compare  this  with  the  local  mean  time  of  the 
individual  observation,  the  difference  in  times  will  give  the 
longitude  of  the  observer  from  Greenwich.  This  is  a  method 
much  in  vogue  for  the  determination  of  longitude  when  no 
great  accuracy  is  necessary,  for  such  an  eclipse  is  a  gradual 
phenomenon  and  not  one  which  comes  with  great  sudden- 
ness. The  Greenwich  mean  times  of  the  phases  of  an  eclipse 
of  the  moon  are  the  same  for  every  inhabitant  of  the  earth, 
and  consequently,  in  each  of  the  annual  nautical  almanacs 
it  is  possible  to  publish  the  Greenwich  times  of  the  various 


64  ECLIPSES    OF    THE    SUN 

portions  of  the  eclipse,  such  as  the  beginning  of  the  partial 
eclipse,  the  beginning  of  the  total  eclipse,  etc.  If  the  person 
observing  the  lunar  eclipse  lives  in  Western  Europe,  in 
Great  Britain,  France,  Spain,  Portugal,  Holland,  Belgium 
or  any  of  the  other  countries  that  now  keep  Greenwich  time 
as  standard,  the  times  of  the  eclipse  will  be  that  recorded 
by  their  clocks,  if  these  keep  correct  time.  If  the  observer 
lives  in  New  York  or  Washington  or  anywhere  in  United 
States  or  Canada  where  Eastern  Standard  time  is  used,  the 
times  of  the  eclipse  will  be  obtained  by  subtracting  5  h.,  o 
m.,  o  s.  from  the  Greenwich  time,  while  if  he  lives  in  San 
Francisco  it  will  be  necessary  to  subtract  8  h.  from  the 
Greenwich  time. 

The  method  of  calculating  the  times  of  a  lunar  eclipse, 
as  given  in  the  ephemerides,  is  a  beautifully  attractive  prob- 
lem which  may  be  solved  quite  simply  by  anyone  even 
though  he  is  not  an  expert  computer,  and  this  may  be  ac- 
complished either  by  graphical  methods  or  by  logarithmic 
calculation.  Since  the  earth  has  an  atmosphere,  the  outline 
of  its  shadow  is  not  sharp  and  well-defined,  but  hazy  when 
examined  by  a  telescope.  On  account  of  the  indefinite  out- 
line of  the  earth's  shadow,  its  theoretical  diameter  must  be 
augmented.  Different  computers  utilize  different  values  for 
this  increase,  varying  from  one-fiftieth  to  one-seventy-fifth 
of  the  theoretical  value  of  the  diameter  of  the  earth's 
shadow.  Accordingly,  a  total  lunar  eclipse  is  not  a  sudden 
phenomenon  like  an  eclipse  of  the  sun  and  there  is,  there- 
fore, no  need  to  employ  very  refined  methods  of  computa- 
tion, or  to  carry  the  calculations  to  fractions  of  a  second 
of  time.  The  American  Ephemeris  gives  the  times  to  the 
decimals  of  a  minute,  —  an  accuracy  sufficiently  great  for 
the  purpose.  The  method  of  calculating  a  lunar  eclipse  is 
given  in  Chauvenet,  Spherical  and  Practical  Astronomy, 
Vol.  i,  p.  589.  For  graphical  methods  of  determining  the 
times  of  eclipses  of  moon  and  sun,  and  also  of  occultations, 
see  a  series  of  articles  by  Rev.  W.  F.  Rigge  in  Popular  As- 
tronomy, volume  III. 

As  just  stated,  an  eclipse  of  the  moon  does  not  permit  of 
an  exact  calculation  owing  to  the  ill-defined  nature  of  the 


THE    VERIFICATION    OF    ECLIPSES  65 

earth's  shadow  caused  by  its  atmosphere.  Due  to  the  re- 
fraction of  light  by  the  earth's  atmosphere,  the  sun's  light, 
or  rather  the  rays  at  the  red  end  of  the  spectrum  that  are 
not  absorbed  by  the  atmosphere,  reach  the  moon,  even 
at  the  middle  of  totality  when  the  earth  is  directly  between 
the  moon  and  the  sun.  If  the  "  man  in  the  moon  "  were 
to  view  the  phenomenon  that  would  form,  to  him,  an  eclipse 
of  the  sun,  he  would  see  the  earth  surrounded  by  a  great 
ring  of  light,  its  own  illuminated  atmosphere.  If  the  moon 
had  even  a  rare  atmosphere,  there  would,  for  this  same 
reason,  be  a  ring  of  light  around  the  moon  as  it  en- 
croached upon  the  face  of  the  sun  at  the  time  of  a 
solar  eclipse.  No  such  light  is  visible,  which  is  a  sure 
proof  that  any  atmosphere  the  moon  may  possess  must  be 
rarer  than  can  be  made  by  the  best  vacuum  pump  ever  con- 
structed by  man.  The  coming  of  totality  at  the  time  of  an 
eclipse  of  the  sun  is  a  sudden  phenomenon  and  as  a  conse- 
quence the  approximate  methods  utilized  in  calculating  a 
lunar  eclipse  are  not  sufficiently  precise.  The  best  method 
to  follow  in  determining  the  times  of  a  total  eclipse  is  that 
of  Chauvenet,  Spherical  and  Practical  Astronomy,  i,  436. 
Another  excellent  guide  is  Buchanan,  Theory  of  Eclipses. 

The  calculation  of  a  solar  eclipse,  in  fact,  cannot  be 
treated  in  the  simple  manner  of  that  of  a  lunar  eclipse  owing 
to  the  large  size  of  the  moon's  parallax.  In  other  words,  the 
moon  is  so  comparatively  near  the  earth  that  it  is  projected 
on  the  face  of  the  sun  differently  for  every  separate  place  on 
the  earth's  surface.  As  a  result,  the  times  of  beginning  and 
ending  of  the  partial  and  total  phases  of  the  eclipse  are  dif- 
ferent at  every  station. 

The  accuracy  with  which  the  times  of  solar  eclipses  can 
be  predicted  depends  on  the  reliability  of  the  work  of  the 
astronomers  of  all  ages,  and  on  the  manner  in  which  the 
torch  has  been  passed  on  from  one  generation  to  the  next. 
The  chief  cause  of  concern  is  found  in  the  motions  of  our 
unruly  neighbor,  the  moon.  The  position  of  the  moon  is 
furnished  from  observations  of  the  times  of  contact  of  the 
limbs  of  the  sun  and  moon,  four  different  contacts  being 
recognized.  First  contact  is  the  instant  that  the  moon  be- 


66  ECLIPSES    OF   THE    SUN 

gins  to  creep  on  the  face  of  the  sun,  the  eclipse  beginning  on 
the  western  edge  of  the  sun.  Second  contact  is  signaled 
by  the  beginning  of  totality,  third  contact  by  the  ending  of 
totality  and  fourth  contact  is  the  ending  of  the  eclipse,  the 
moon  passing  off  the  face  of  the  sun,  the  last  contact  being 
found  on  its  eastern  edge.  The  four  contacts  are  generally 
observed  visually  by  a  pair  of  field  glasses  or  by  a  telescope 
of  moderate  power.  The  time  of  first  contact  is  difficult  to 
observe  with  accuracy  since  nothing  is  to  be  seen  at  the  edge 
of  the  sun  until  the  moon  is  actually  projected  on  to  the  face 
of  the  sun,  —  and  first  contact  has  already  actually  taken 
place.  In  other  words,  the  observer  is  always  too  late  in 
noting  the  time  of  first  contact,  the  amount  of  tardiness 
depending  on  the  size  of  the  telescope,  the  state  of  the  see- 
ing, but  especially  on  the  skill  and  experience  of  the  ob- 
server, which  in  turn  depend  on  the  number  of  eclipses  wit- 
nessed. Fourth  contact  is  easier  to  observe  than  first  since 
the  moon  can  be  followed  in  the  telescope  until  it  leaves  the 
face  of  the  sun.  The  beginning  and  ending  of  totality  can  be 
more  accurately  observed  than  the  two  other  contacts,  but 
they  are  subject  to  some  uncertainties  on  account  of  irregu- 
larities in  the  profile  of  the  moon  at  the  points  of  contact. 
By  means  of  photographs  taken  just  before  and  just  after 
totality,  when  the  crescent  of  the  sun  is  small  and  changing 
rapidly,  the  times  of  second  and  third  contacts  can  be  deter- 
mined with  much  greater  degree  of  precision  than  is  attain- 
able in  visual  work.  It  goes  without  saying  that  the  error 
of  the  chronometer  should  be  known  as  accurately  as 
possible  (probably  obtained  by  wireless  signals),  that  the 
observed  times  should  be  recorded  on  the  chronograph,  and 
that  the  latitude  and  longitude  be  known. 

The  photographs  for  contacts  will  unquestionably  be 
taken  with  the  same  instrument  used  during  totality  for  the 
corona.  It  will  be  necessary,  therefore,  to  utilize  a  different 
method  of  exposure  and  different  technique  from  that  em- 
ployed on  the  corona.  The  brilliancy  of  the  crescent  sun 
compared  with  the  corona  will  be  very  great.  To  diminish 
the  brightness  of  the  crescent  images  it  will  be  necessary  to 
use  a  rapid  exposing  shutter.  The  best  results  photographi- 


THE    VERIFICATION    OF    ECLIPSES  67 

cally  will  be  secured  by  using  slow,  fine-grained  plates,  and 
they  should  be  specially  "  backed  "  to  prevent  halation 
caused  by  reflection  from  the  glass  side  of  the  plate.  (See 
Chapter  VII.) 

At  the  eclipse  of  1905,  totality  came  ahead1  of  its  pre- 
dicted time,  the  beginning  of  totality  being  17  seconds 
earlier,  while  the  ending  came  23  seconds  earlier  than  the 
times  predicted  by  the  American  Ephemeris.  The  middle 
of  totality  was  thus  20  seconds  ahead  of  that  calculated, 
while  the  duration  of  totality  was  some  six  seconds  less  than 
was  expected  from  the  computations.  The  time  predicted 
for  the  middle  of  totality  by  the  British  Nautical  Almanac 
was  identical  with  that  furnished  by  the  American  Ephem- 
eris but  the  duration  of  the  former  was  is.7  less  than  that 
of  the  latter,  while  the  duration  calculated  from  the  Con- 
naissance  des  Temps  was  five  seconds  greater  than  that  of 
the  American  Ephemeris.  All  observers  at  the  Spanish 
eclipse  had  their  program  of  observation  greatly  interfered 
with  by  having  the  moon  so  far  in  advance  of  its  predicted 
place.  Before  the  eclipse  of  June,  1918,  the  observers  of 
the  U.  S.  Naval  Observatory  party  in  Oregon  were  furnished 
by  the  Washington  authorities  with  a  correction  of  12.5 
seconds  to  be  applied  to  the  times  of  contacts  calculated 
from  the  American  Ephemeris.  The  observed  times  were 
about  fourteen  seconds  ahead  of  those  predicted  by  the 
Ephemeris.  At  the  eclipse  of  1922,  the  Lick  Observatory 
party  observed  the  beginning  of  totality  some  sixteen 
seconds  earlier  than  the  time  predicted,  while  the  end  of 
totality  came  twenty  seconds  earlier  than  the  Almanac 
prediction. 

Apparently,  therefore,  after  having  made  due  allowance 
for  all  known  possible  sources  of  error,  the  moon  has  strayed 
from  the  path  mapped  out  for  it  by  the  mathematical  astron- 
omers by  an  amount  which  is  not  inconsiderable.  What 
is  the  cause  of  the  moon  being  away  from  its  predicted 
place? 

The  first  of  the  great  modern  authorities  dealing  with  the 

1  Lick  Observatory  Bulletin,  4,  118,  1905. 


68  ECLIPSES    OF    THE    SUN 

motion  of  the  moon  was  Hansen,  who,  in  1857,  published 
his  Tables  de  la  Lune.  By  the  help  of  these  tables  the  un- 
explained fluctuations  in  the  moon's  motion  were  reduced 
to  a  small  fraction  of  their  former  amount.  Hansen  de- 
rived the  inequalities  in  the  moon's  motion  largely  from  the 
gravitational  theory,  but  in  order  to  satisfy  the  observa- 
tions made  with  great  care  at  Greenwich  between  1750  and 
1850,  it  was  necessary  for  him  to  apply  two  empirical  terms 
supposed  to  come  from  Venus,  the  larger  of  the  two  having 
a  long  period  of  239  years. 

The  next  advance  was  made  by  Simon  Newcomb,  who 
published  his  Researches  on  the  Motion  of  the  Moon  in 
1878.  In  addition  to  the  observational  material  utilized  by 
Hansen,  Newcomb  discussed  all  of  the  lunar  eclipses  re- 
corded by  Ptolemy  in  the  Almagest  and  in  addition  a  large 
number  of  mediaeval  lunar  eclipses.  He  discussed  a  few 
ancient  solar  eclipses,  but  excluded  them  from  his  calcula- 
tions on  account  of  their  unreliability.  The  chief  value  of 
Newcomb 's  great  work  lay  in  collecting  and  discussing 
observations  of  occultatioris  and  solar  eclipses  in  the  cen- 
tury and  a  quarter  previous  to  1750.  Newcomb  had 
thus  250  years  of  observations  to  discuss  in  place  of 
the  hundred  years  of  observations  available  to  Hansen. 
The  whole  material  was  utilized  to  secure  the  value  of  the 
moon's  motion  and  acceleration,  and  clearly  showed  the 
presence  of  an  unexplained  term  of  long  period.  Newcomb 
did  not  deduce  the  period  of  this  term  from  the  observations 
themselves  but  assumed  it  identical  with  one  of  Hansen 's 
terms  (which  is  now  known  to  be  faulty).  "  It  is  no  small 
tribute  to  the  thoroughness  of  Newcomb 's  work  that  while 
the  observations  from  1750  onwards  have  been  thoroughly 
examined  by  Cowell,  Radau,  and  Brown,  the  reductions  of 
the  observations  from  1621  to  1747  have  never  been  revised 
by  anyone  but  Newcomb  himself."  x 

The  third  great  investigation  of  the  lunar  theory  is  found 
in  E.  W.  Brown's  Tables  of  the  Moonys  Motion,  published 
in  1920.  These  tables  are  more  complete  than  those  of 
Hansen  and  Newcomb,  and  take  account  of  every  term  of 

1  Fotheringham,  Monthly  Notices,  R.  A.  S.,  80,  289,  1920. 


THE    VERIFICATION    OF    ECLIPSES  69 

appreciable  significance.  In  addition  to  these  three  great 
monumental  works  on  the  Theory  of  the  Moon  by  Hansen, 
Newcomb  and  Brown,  exhaustive  investigations  have  been 
made  by  many  competent  authorities.  After  making  allow- 
ance for  the  gravitational  attraction  of  every  conceivable 
form  of  disturbance,  it  is  unmistakable  that  the  moon  de- 
Darts  from  her  theoretical  place  in  a  very  irregular  manner. 
An  error  in  the  assumed  value  of  the  acceleration  of  the 
moon's  mean  longitude,  even  though  this  error  is  a  very  small 
3ne,  will  cause  errors  in  the  predicted  place  of  the  moon  which 
increase  in  size  for  the  reason  that  the  errors  depend  on  the 
square  of  the  elapsed  time. 

Tables  of  the  Moon  are  used  for  predicting  the  place  of 
the  moon  in  the  different  nautical  almanacs  which  appear 
some  two  or  three  years  in  advance.  For  these  predictions 
It  is  desirable  to  keep  the  theory  of  the  moon  as  free  as 
Dossible  from  arbitrary  empirical  terms  so  that  the  theory 
nay  not  be  cluttered  up  by  too  many  additions.  When  ob- 
servations are  secured  the  results  can  then  be  directly  com- 
pared with  theory.  Between  the  theoretical  position  of  the 
noon  given  by  the  almanac,  and  the  observed  place,  there 
nay  be  a  difference  of  several  seconds  of  arc. 

For  guiding  the  work  of  the  eclipse  astronomer  it  is  now 
leemed  necessary  to  know  the  times  of  beginning  and  ending 
)f  a  total  solar  eclipse  within  an  error  of  a  few  seconds, 
rlence  it  has  become  customary  in  recent  years  to  secure 
Tom  Washington,  or  Greenwich,  a  month  or  more  before  the 
eclipse  takes  place,  corrections  to  the  almanac  positions  of 
;he  moon.  These  corrections  derived  from  observations  in 
he  three-year  interval  since  the  almanac  was  prepared  per- 
nit  increased  accuracy  in  computing  the  eclipse. 

On  account  of  the  difficulty  of  seeing  the  moon  when  near 
he  time  of  new  moon,  there  is  ordinarily  a  gap  in  the  regular 
unar  observations,  whether  these  are  made  by  occultations 
>r  by  meridian  circle.  An  intense  interest  was  aroused  in 
he  United  States  by  the  total  eclipse  of  1925  and  hence  a 
pecial  attempt  was  made  to  utilize  this  interest  in  order  to 
iecure  as  many  observers  as  possible.  From  large  numbers 
>f  careful  observations  it  was  hoped  that  it  might  be  pos- 


70 


ECLIPSES    OF    THE    SUN 


sible  to  detect  any  short-period  deviations  in  the  motion  of 
the  moon,  as  well  as  to  compare  the  results  obtained  by  dif- 
ferent methods  of  observation  in  order  to  see  whether  there 
are  systematic  errors  of  observation  peculiar  to  any  of  them. 
At  the  time  of  the  1925  eclipse  it  was  found  that  the  dif- 
ferences in  the  observed  and  tabular  positions  of  the  moon 
had  been  nearly  constant  during  the  interval  since  the 
Ephemeris  was  published.  For  predicting  the  eclipse,  it  was 
necessary  to  apply  a  correction  of  +  7^.0  to  the  tabular 
mean  longitude  of  the  moon  as  given  in  the  American 
Ephemeris  and  — 0^.50  to  the  lunar  declination.  After  ap- 
plying these  changes  to  the  moon's  place,  the  observations  of 
the  beginning  and  ending  of  totality  gave  the  following  cor- 
rections to  be  applied  to  the  predicted  times  of  second  contact 
and  duration  of  totality,  the  observations  coming  from 
trained  astronomers  with  every  facility  for  obtaining  accu- 
rate positions  of  their  stations  and  accurate  time  signals  on 
the  day  of  the  eclipse. 


Place 

Poughkeepsie,  N.  Y. 
Beacon,  N.  Y. 
New  Haven,  Conn. 
Middletown,  Conn. 
Martha's  Vineyard,  Mass. 
Nan  tucket,  Mass. 
Dirigible,  Los  Angehs 


Second 
Contact 

+  3-M 
+  i   8 

4-  ()  o 

+  3  6 
+  60 

+  49 


Duration 

—  o.s8 

-  2    Q 
+  08 

O    O 

—  6. 

-  3   7 

-  0.9 


A  summary  of  the  results  of  the  observations  discussed  by 
Brown1  for  the  1925  eclipse  gives  corrections  to  be  added  to 
the  mean  longitude  of  the  moon  8X,  and  to  the  mean  latitude 
S/3  as  follows,  where  n  represents  the  number  of  observations: 


Method 

6X  —  7  "oo 

n 

S0 

n 

Occultations 

+  o  "38  ±  ."10 

79 

+  o  "40  db    "17 

7Q 

Photographs 

—  o  14  ±  .25 

ir 

+  o  34  dc  .  26 

i  i 

Greenwich,  meridian  circle 

—  o  35  dc  .16 

16 

—  0   85  rb   .11 

16 

Washington,  g-inch  transit 

-  o  34  ±    15 

16 

—  o  60  ±  .16 

16 

Washington,  6-inch  transit 

+  0    70  ±      IQ 

22 

—  0.13  ±  .18 

21 

Cape,  meridian  circle 

+  o  42  d=    28 

J3 

—  o  34  dr  .46 

TO 

Eclipse 

-f-  O   40  dr      14 

+  0    80  ±    .12 

1  Astronomical  Journal,  37,  9,  1926. 


THE    VERIFICATION    OF    ECLIPSES  71 

The  weighted  mean  of  the  results  gives  a  correction  to  the 
moon's  longitude  and  latitude  of  +  0.^28  ±."06  and 
+  o."32  ±."06,  respectively. 

The  comparison  of  the  results  made  by  the  different 
methods  is  not  without  interest.  The  meridian  observations 
differ  rather  widely.  The  occultations  and  eclipse  results 
give  corrections  which  agree  within  their  probable  errors. 
All  the  non-meridian  observations  agree  in  giving  a  positive 
correction  to  the  lunar  latitude  and  all  the  meridian  results 
give  a  negative  correction,  and  hence  systematic  differences 
of  over  i."o  are  shown.  These  discrepancies  are  partially  ex- 
plained by  an  assumption  originally  made  by  Hansen  that 
there  is  a  difference  between  the  center  of  mass  and  center  of 
figure  of  the  moon.  Brown  has  shown  that  the  northern  half 
of  the  moon  is  probably  the  denser  half. 

The  ranges  in  the  times  of  second  contact  and  the  duration 
of  the  eclipse,  as  exhibited  in  the  table  above,  based  on  ob- 
servations of  skilled  astronomers,  appear  much  larger  than 
one  would  expect,  but  the  observations  of  contacts  during 
the  eclipse  of  1914  contained  in  Meddelande  jran  Lunds 
Astron.  Obs.,  No.  20,  show  about  the  same  differences.  How- 
ever, it  must  not  be  forgotten  that  the  outline  of  the  moon 
departs  from  a  perfect  circle  and  that  the  time  of  the  begin- 
ning and  ending  of  a  total  solar  eclipse  depends  very  largely 
on  the  character  of  the  lunar  surface  at  the  point  of  contact, 
the  beginning  of  the  total  eclipse  not  taking  place  until  the 
last  Baily  bead  has  disappeared. 

The  marvelous  accuracy  in  the  prediction  of  eclipses  shows 
the  wonderful  precision  and  perfection  of  the  science  of  as- 
tronomy. An  observation  of  an  eclipse  of  the  sun  made  three 
thousand  years  ago  has  an  important  bearing  on  the  most 
recent  refined  researches  on  the  motion  of  the  moon ! 


CHAPTER    V 

THE   SPECTROSCOPE 

ASTROPHYSICS,  called  the  "  new  astronomy/7  has 
revealed  in  a  remarkable  manner  through  its 
discoveries  during  the  past  half  century  the  won- 
derful ability  and  resourcefulness  of  the  human  brain,  mak- 
ing it  evident  that  man  is  gifted  with  almost  infinite  powers. 
From  this  earth  of  ours,  which  astronomy  teaches  us  to  be 
but  an  insignificant  speck  among  the  countless  orbs  of  the 
universe,  man  has  been  able  to  reach  out  and  ascertain  the 
physical  constitution  of  the  sun,  and  to  acquire  this  in- 
formation with  almost  the  certainty  of  a  chemist  who  could 
make  a  qualitative  analysis  provided  that  an  average  speci- 
men of  the  sun's  matter  could  be  furnished  him.  And 
across  millions  and  millions  of  miles  of  space,  the  far  dis- 
tant suns,  the  stars,  shining  by  their  own  feeble  light,  are 
made  to  give  up  the  secrets  of  their  construction.  Not 
only  can  we  learn  of  the  constitution  of  the  sun  and  stars, 
but  we  can  also  ascertain  their  effective  temperatures  as 
well,  and  with  information  thus  garnered  we  can  arrange 
the  stars  in  an  orderly  sequence,  tracing  their  evolution 
from  the  swollen  red  stars  of  very  minute  density,  the  so- 
called  "  giants/7  through  the  successive  stages  of  yellow 
and  white  to  that  of  the  stars  of  class  B,  and  then  on  in 
the  descending  branch  of  development  as  the  stars  become 
cooler  and  more  dense.  The  astronomer  believes  in  evolu- 
tion and  recent  researches  make  evident  that  our  gigantic 
sun  is  but  a  yellow  "  dwarf/7  well  advanced  towards  the 
state  of  old  age  and  final  obscurity.  It  is  by  means  of  the 
spectroscope  that  such  information  is  gathered,  and  by  the 
spectroscope  it  has  become  possible  to  investigate  motions, 
not  athwart  the  sky  as  the  older  astronomy  was  able  to  do, 
but  towards  us  or  away  from  us  in  the  line  of  sight,  and 
to  measure  these  motions  in  miles  per  second.  The  shift 

72 


THE    SPECTROSCOPE  73 

of  the  lines  of  the  spectrum  due  to  motion  in  the  line  of 
sight,  which  has  been  confirmed  experimentally  in  the 
laboratory,  has  given  rise  to  many  interesting  developments 
in  astrophysics:  the  discovery  of  an  entirely  new  class  of 
bodies,  spectroscopic  binaries,  the  measurement  of  the 
axial  rotation  of  the  sun  and  Jupiter,  as  well  as  providing  a 
magnificent  confirmation  of  the  meteoric  constitution  of 
Saturn's  rings.  And  quite  recently  the  greatest  triumph 
of  the  spectroscope  has  been  achieved  by  Adams  and  his 
co-workers  at  Mount  Wilson  Observatory  being  able  to 
determine  the  distances  of  the  stars  and  thus  find  their 
luminosities  compared  with  that  of  the  sun.  Since  its 
birth  in  1859,  when  Kirchhoff  discovered  the  principles  of 
spectrum  analysis,  astrophysics  has  advanced  by  leaps  and 
bounds.  In  no  branch  of  astronomy  has  the  new  instru- 
ment of  research  shown  its  outstanding  value  quite  as  clearly 
as  in  the  development  of  the  subject  of  eclipses  of  the  sun. 
For  a  clear  understanding  of  the  matter  it  will  therefore 
be  necessary  to  give  a  brief  account  of  the  history  of  the 
new  astronomy. 

The  scientific  world  owes  much  to  John  Kepler  for  hand- 
ing down  to  us  the  three  great  laws  of  planetary  motion,  but 
his  activities  extended  beyond  the  realm  of  mathematical 
astronomy  into  the  domain  of  physics.  He  was  the  first  to 
show  —  as  he  did  in  his  "  Dioptrics  "  —  that  if  a  beam 
of  sunlight  be  allowed  to  fall  upon  a  prism  in  a  certain  way 
it  passes  out  as  a  colored  beam,  giving  the  colors  of  the 
rainbow.  And  as  the  great  Sir  Isaac  Newton  took  Kepler's 
Laws  and  used  them  for  finding  the  law  of  gravitation,  so 
likewise  he  extended  the  physical  work  of  Kepler  and  by 
reason  and  experimentation  greatly  increased  our  knowledge. 
In  fact,  it  is  from  Newton's  labors  that  the  science  of  spec- 
trum analysis  virtually  had  its  birth. 

In  1666,  Newton,  by  allowing  sunlight  to  pass  through  a 
hole  in  a  shutter  and  to  fall  on  a  glass  prism,  found  a  colored 
ribbon  of  light,  an  impure  spectrum.  His  description  of  the 
effect  observed  is  so  clear  that  the  following  is  copied  from 
Optics,  Third  edition,  page  21." 

1  See  also,  The  Spectroscope  and  its  Work,  Newall,  191 1.  For  greater  details 
consult  Chemistry  of  the  Sun,  Lockyer,  1887,  and  Handbuch  der  Spectroscopie, 
Kavser,  Vol.  i. 


74  ECLIPSES    OF    THE    SUN 

"  In  a  very  dark  chamber  at  a  round  hole  about  one- 
third  part  ot  an  inch  broad  made  in  the  shut  of  a  window 
I  placed  a  glass  prism,  whereby  the  beam  of  the  sun's  light 
which  came  in  at  that  hole  might  be  refracted  upwards 
toward  the  opposite  wall  of  the  chamber,  and  there  form  a 
coloured  Image  of  the  Sun.  The  axis  of  the  prism  (that  is 
the  line  passing  through  the  middle  of  the  prism  from  one 
end  of  it  to  the  other  end  parallel  to  the  edge  of  the  refract- 
ing angle)  was  in  this  and  the  following  experiments  per- 
pendicular to  the  incident  rays.  About  this  axis  I  turned 
the  prism  slowly,  and  saw  the  refracted  light  on  the  wall 
or  coloured  image  of  the  sun  first  to  descend,  and  then  to 
ascend.  Between  the  descent  and  ascent  when  the  image 
seemed  stationary,  I  stopped  the  prism  and  fix'd  it  in  that 
posture,  that  it  should  be  moved  no  more.  For  in  that 
posture  the  refractions  of  the  light  at  the  two  sides  of  the 
refracting  angle,  that  is  at  the  entrance  of  it,  were  equal  to 
one  another.  So  also  in  other  experiments,  as  often  as  I 
would  have  the  refractions  on  both  sides  of  the  prism  to  be 
equal  to  one  another,  I  noted  the  place  where  the  image  of 
the  sun  formed  by  the  refracted  light  stood  still  between  its 
two  contrary  motions,  in  the  common  period  of  its  progress 
and  regress;  and  when  the  image  fell  upon  that  place,  I 
made  fast  the  prism.  And  in  this  posture,  as  the  most  con- 
venient, it  is  to  be  understood  that  all  the  prisms  are  placed 
in  the  following  experiments,  unless  where  some  other  pos- 
ture is  described.  The  prism  therefore  being  placed  in  this 
posture,  I  let  the  refracted  light  fall  perpendicularly  upon 
a  sheet  of  white  paper  at  the  opposite  wall  of  the  chamber, 
and  observed  the  figure  and  dimensions  of  the  solar  image 
formed  on  the  paper  by  the  light.  This  image  was  oblong 
and  not  oval,  but  terminated  with  two  rectilinear  and  paral- 
lel sides,  and  two  semicircular  ends.  On  its  sides  it  was 
bounded  pretty  distinctly,  but  on  its  ends  very  confusedly 
and  indistinctly,  the  light  there  decaying  and  vanishing  by 
degrees." 

Newton  concluded  that  white  light  was  made  up  of 
separate  colored  rays;  by  passing  the  light  through  the 
prism,  the  different  rays  suffered  different  amounts  of  re- 


THE    SPECTROSCOPE  75 

fraction;  they  were  in  fact  dispersed,  and  as  a  result  the 
spectrum  consisted  of  an  infinite  number  of  colored  images 
of  the  round  hole  lying  side  by  side.  In  the  middle  of  the 
spectrum  the  different  rays  overlapped,  and  white  light 
resulted,  but  the  ends  remained  colored.  Newton  found 
that  by  the  use  of  a  slit  "  an  inch  or  two  long,  and  a  tenth 
or  a  twentieth  of  an  inch  in  width,'7  the  spectrum  became 
purer;  and  he  even  tried  a  triangular  hole,  observing 
greater  and  greater  purity  as  the  vertex  was  approached. 
He  proved  conclusively  that  the  colors  came  from  the  sun's 
light  itself  and  not  from  the  prisms,  for  he  produced  spectra 
with  a  variety  of  different  prisms,  and  afterwards  combined 
the  prismatic  colors  together  to  make  white  light.  He 
further  showed  that  each  ray  of  light  consisted  of  a  single 
color  and  possessed  a  certain  definite  refrangibility. 

Newton's  books  on  optics  (published  in  1704)  are  mar- 
vels of  clearness  of  exposition,  and  his  fundamental  ex- 
periments have  come  down  to  us  almost  unchanged.  The 
significant  result  of  Newton's  work  is  that  the  pure  colors, 
and  not  white  light,  occupy  the  primordial  position  of  im- 
portance, since  it  is  possible  to  form  all  conceivable  colors  in- 
cluding white  light  from  a  mixture  of  the  pure  colors.  A 
determined  color  in  the  spectrum  is  defined  through  its  index 
of  refraction,  and  since  this  quantity  continually  varies, 
there  is  an  infinite  number  of  spectral  colors. 

Newton  did  not  see  any  of  the  dark  lines  of  the  solar 
spectrum,  now  known  as  Fraunhofer  lines,  though  he  used 
a  narrow  slit  and  should  have  been  able  to  see  them,  —  but 
his  prisms  were  poor  (as  he  himself  said).  The  authority 
of  his  great  name  discouraged  further  experimentation,  and 
no  advances  were  made  for  one  hundred  years.  Unfortu- 
nately he  made  some  mistakes,  first  in  dividing  the  colors 
into  seven,  the  quantitative  number  of  such  great  attrac- 
tion and  "  perfection  "  to  the  early  scientists,  thereby  per- 
haps preventing  the  discovery  of  the  Fraunhofer  lines,  and 
second,  in  not  seeing  that  various  media  dispersed  differ- 
ently. As  is  well  known,  Newton  said  that  the  case  of  the 
refracting  telescope  was  a  deplorable  one,  and  following 
his  time  these  telescopes  were  made  of  enormous  lengths, 


76  ECLIPSES    OF    THE    SUN 

up  to  two  hundred  or  even  three  hundred  feet,  in  order  to 
minimize  the  color  difficulty.  However,  if  Newton  had 
been  in  a  position  to  make  use  of  different  kinds  of  glass 
of  crown  and  flint  as  Fraunhofer  was  later,  it  is  altogether 
likely  that  many  other  discoveries  of  great  importance 
would  have  been  made  by  him. 

The  long  period  of  arrested  progress  was  broken  in  1802 
when  Wollaston,  making  use  of  a  slit,  repeated  Newton's 
experiments.  He  then  found,1  "  The  colours  into  which  a 
beam  of  white  light  is  separated  by  refraction  appears 
to  me  to  be  neither  seven,  as  they  are  usually  seen  in  the 
rainbow,  nor  reducible  by  any  means  to  three,  as  some 
persons  have  conceived;  but  that,  by  employing  a  very 
narrow  pencil  of  light,  four  primary  divisions  of  the 
prismatic  spectrum  may  be  seen  with  a  degree  of  distinct- 
ness that,  I  believe,  has  not  been  described  nor  observed 
before.  .  .  .  The  four  colors  are,  red,  yellowish-green,  blue 
and  violet."  He  then  goes  on  to  describe  the  dark  markings 
that  were  seen  in  his  spectrum,  these  being  ill-defined  in 
appearance  and  evidently  taken  as  the  natural  divisions 
between  the  colors.  It  is  readily  apparent  that  the  prisms 
used  by  Wollaston  were  likewise  poor  in  quality,  for  his 
experiments  were  made  in  a  manner  that  should  have 
brought  to  view  the  thousands  of  dark  lines  in  the  solar 
spectrum.  However,  a  very  great  advance  was  made  over 
the  work  of  Newton,  for  the  spectrum  of  a  candle  flame 
and  of  an  electric  light  were  examined.  Spectra  were  found 
which  not  only  differed  from  each  other  in  appearance  but 
each  of  which  was  entirely  unlike  that  furnished  by  the 
sun.  The  candle  and  the  electric  light  each  gave  a  spec- 
trum of  bright  lines,  a  discontinuous  spectrum.  The  funda- 
mental importance  of  these  experiments  was  not  at  the  time 
recognized. 

The  life  of  Joseph  Fraunhofer2  had  an  almost  tragic 
beginning.  At  the  age  of  fourteen  he  lived  in  a  dilapidated 
house  in  an  alley  in  Munich  which  tumbled  down  and 
buried  its  occupants  in  the  ruins.  The  other  residents  were 

1  Phil.  Trans.,  part  i,  378,  1802. 

2  Clerke,  History  of  Astronomy  during  the  Nineteenth  Century. 


THE    SPECTROSCOPE  77 

killed,  but  the  boy,  who  was  an  orphan,  was  dragged  out. 
more  dead  than  alive  and  seriously  injured.  The  Elector 
Maximilian  Joseph  was  a  witness  of  the  accident  and  to 
show  his  commiseration  made  him  a  present  of  eighteen 
ducats.  Besides  the  purchase  of  books  and  of  a  glass- 
grinding  machine  the  money  permitted  his  release  from  ap- 
prenticeship to  a  looking-glass  maker.  Through  study  and 
toil  and  privation  he  increased  his  knowledge  of  the 
optician's  art  and  at  the  age  of  nineteen  entered  the  glass- 
making  firm  of  Von  Reichenbach  and  Utzschneider.  He  de- 
voted himself  now  with  great  avidity  to  a  study  of  lenses 
for  the  purpose  of  improving  the  refracting  telescope.  After 
Newton's  time,  Dollond  had  discovered  that  it  was  possible 
to  banish  most  of  the  color  which  so  interfered  with  the 
action  of  the  refracting  telescope,  by  combining  a  lens  of 
crown  glass  with  one  of  flint.  By  means  of  many  experi- 
ments with  prisms  of  glass  of  different  varieties,  Fraunhofer 
was  able  to  investigate  the  best  combination  of  two  lenses 
that  would  give  the  most  perfect  definition  with  freedom 
from  the  disturbing  color.  In  1817  there  was  finished  the 
great  "  Dorpat  refractor  "  with  the  then  unprecedented 
aperture  of  nine  and  a  half  inches.  This  telescope  in  the 
skillful  hands  of  Struve  became  one  of  the  most  famous 
telescopes  ever  in  existence.  To  Fraunhofer  the  astronomi- 
cal world  is  also  indebted  for  the  first  really  serviceable 
heliometer,  that  at  Konigsberg,  an  instrument  which  was  to 
play  an  important  part  in  extending  our  knowledge  of 
the  sidereal  universe  by  permitting  the  measurement  of 
stellar  distances,  or,  as  they  are  technically  called,  stellar 
parallaxes. 

In  1814,  Fraunhofer  not  only  extended  Wollaston's  work 
but  introduced  great  improvements  in  the  method  of  ob- 
serving. The  slit  was  retained  but  placed  at  a  great 
distance  from  the  prism.  Instead  of  allowing  the  refracted 
beam  of  light  to  fall  on  a  screen  he  placed  a  small  telescope 
directly  behind  the  prism  and  by  this  means  a  magnified 
view  of  the  spectrum  was  obtained. 

"  Into  a  dark  room,  and  through  a  vertical  aperture  in 
the  window-shutter,  about  15"  broad  and  36"  high,  I  intro- 


78  ECLIPSES    OF    THE    SUN 

duced  the  rays  of  the  sun  upon  a  prism  of  flint  glass  placed 
upon  the  theodolite;  this  instrument  was  24  feet  from  the 
window,  and  the  angle  of  the  prism  was  nearly  60°.  The 
prism  was  placed  before  the  object  glass  of  the  telescope, 
so  that  the  angles  of  incidence  and  emergence  were  equal. 
In  looking  at  this  spectrum  for  the  bright  line  which  I  had 
found  in  the  spectrum  of  the  artificial  light,  I  discovered, 
instead  of  this  line  an  infinite  number  of  vertical  lines  of 
different  thickness.  These  lines  are  darker  than  the  rest  of 
the  spectrum,  and  some  of  them  appear  entirely  black."  * 

The  interrelation  of  these  lines  and  streaks  appears  to  be 
the  same  no  matter  what  refracting  substance  is  employed 
so  that,  for  instance,  a  particular  band  is  found  in  each  case 
only  in  the  blue,  another  is  found  only  in  the  red,  and  one 
can  therefore  learn  to  recognize  a  particular  line  in  the  spec- 
trum by  noting  its  position  with  respect  to  the  prominent 
lines.  Fraunhofer  observed  that  the  strong  lines  did  not 
mark  the  edges  of  the  colors  as  Wollaston  had  supposed 
and  further  that  the  same  color  is  found  on  both  sides  of  the 
line,  the  colors  grading  by  imperceptible  degrees  from  one 
color  to  the  next.  Starting  from  the  violet  end  of  the  spec- 
trum, the  colors  are  given  the  following  names:  violet,  in- 
digo, blue,  green,  yellow,  orange  and  red.  As  Lockyer  has 
pointed  out,  the  first  letters  of  these  color  names  make  the 
combination  VIBGYOR,  —  an  aid  to  memory  that  the  beginner 
may  find  useful. 

Fraunhofer  constructed  a  map  of  the  lines  of  the  solar 
spectrum,  measuring  by  means  of  the  circle  of  the  theo- 
dolite the  accurate  positions  of  over  350  lines,  though 
he  counted  no  less  than  754.  Starting  at  the  red  end  of  the 
spectrum  he  called  the  more  prominent  lines  by  the  letters 
of  the  alphabet.  Thus  A,  B  and  C  denote  lines  in  the  red 
part  of  the  spectrum,  D  the  prominent  double  lines  in  the 
yellow  part,  which  we  now  know  to  be  due  to  sodium,  while 
H  and  K  in  the  violet  are  the  very  broad  lines  caused  by  cal- 
cium. In  addition,  the  small  letters  of  the  alphabet  were 
made  use  of,  b,  for  instance,  denoting  a  group  of  lines  in  the 
green  due  to  the  element  magnesium.  Not  only  are  the 

1  Denkschriften  der  K.  Akad.  der  Wissen.  zu  Miinchen,  5,  193,   1814. 


THE    SPECTROSCOPE  79 

prominent  lines  of  the  solar  spectrum  to  which  Fraunhofer 
gave  names  called  after  their  discoverer,  but  all  dark  lines 
in  the  spectrum  whether  prominent  or  faint  are  known  as 
Fraunhofer  lines. 

He  plotted  his  lines  not  according  to  their  wave-lengths 
as  in  the  manner  of  all  modern  maps,  but  according  to  a 
rather  arbitrary  scale.  Now  that  the  solar  spectrum  has 
been  more  fully  investigated  with  the  perfected  apparatus 
of  the  twentieth  century,  we  can  go  back  and  gauge  at  its 
true  value  the  worth  of  Fraunhofer's  map.  Thus  we  have 
the  opinion  of  Hartmann  of  Potsdam  to  the  effect  that 
Fraunhofer  made  his  map  with  the  greatest  degree  of  re- 
finement and  secured  a  precision  which  has  warranted  spec- 
troscopists  of  the  past  hundred  years  placing  in  it  the  great 
confidence  that  they  have  felt.  Fraunhofer's  life  and  work 
is  a  splendid  example  of  what  one  man  by  patient  and  care- 
ful work  can  accomplish  for  the  cause  of  science.  Any  one 
who  has  ever  looked  into  a  spectroscope  will  realize  what  a 
colossal  work  the  making  of  this  map  must  have  been. 

Since  the  lines  and  bands  in  the  color  image  have  only  a 
very  small  width,  it  is  evident  that  the  apparatus  must  be 
most  perfect  to  avoid  all  aberrations  which  could  either  ren- 
der the  lines  indistinct  or  entirely  scatter  them.  The  faces 
of  the  prism  must  therefore  be  perfectly  plane;  the  glass 
to  be  used  in  such  prisms  should  be  entirely  free  from  waves, 
streaks  and  striae ;  and  the  greatest  care  should  be  exercised 
in  their  grinding  and  polishing.  These  and  other  considera- 
tions, such  as,  that  the  slit  must  be  parallel  to  the  edge  of  the 
prism,  Fraunhofer  found  out  by  careful  experimenting. 

Fraunhofer,  however,  was  not  content  with  this  work. 
He  wanted  to  know  something  of  the  origin  of  the  lines, 
and  he  soon  came  to  a  conclusion  on  this  point.  It  occurred 
to  him  that  they  might  possibly  be  attributed  to  some  illu- 
sion caused  by  the  narrow  aperture  through  which  the  light 
was  admitted.  We  know  that  the  shape  of  the  slit  has 
something  to  do  with  the  forms  of  these  dark  spaces  in  the 
spectrum,  but  with  their  simple  existence  as  spaces  the  slit 
has  nothing  to  do,  the  mere  shape  of  the  lines  being  quite  a 
trivial  matter.  To  settle  this  question  beyond  doubt  he 


80  ECLIPSES    OF    THE    SUN 

changed  the  slit  which  he  was  using  in  order  to  ascertain 
if  this  would  change  his  spectrum.  He  passed  light  through 
a  small  round  hole  of  15"  in  diameter,  and  allowed  it  to  fall 
upon  the  prism  placed  in  front  of  the  objective  of  the  theod- 
olite. It  is  clear  that  the  color-image  seen  with  the  tele- 
scope can  have  only  an  inappreciable  width,  and  therefore 
will  form  only  a  line;  but  in  this  narrow  colored  width  no 
fine  cross  lines  can  be  seen.  In  order  to  widen  this  narrow 
stretch  of  light  into  a  band  wide  enough  to  see,  Fraunhofer 
made  use  of  a  cylindrical  lens,  or  a  lens  which  is  plane  on 
one  side  and  curved  on  the  other  resembling  a  portion  of  a 
cylinder  of  large  diameter.  The  axis  of  the  cylinder  was 
placed  parallel  to  the  base  of  the  prism  and  hence  parallel 
to  the  line  of  light.  Consequently,  the  width  of  the  spec- 
trum would  be  changed  without  in  any  way  altering  its 
length.  With  this  arrangement  the  lines  were  observed  to 
be  exactly  the  same  as  when  the  light  comes  through  a  long 
narrow  opening.  Hence  the  bands  and  lines  in  the  solar 
spectrum  are  not  caused  by  diffraction  and  interference  by 
the  light  passing  through  the  slit,  nor  are  they  produced  by 
any  peculiarities  in  the  apparatus.  There  is,  therefore,  only 
one  other  possible  cause  for  these  lines,  and  that  is  that  they 
somehow  or  other  belong  to  the  light  given  out  by  the  sun. 
Hence  the  solar  spectrum  is  not  a  continuous  spectrum  having 
light  of  all  colors  and  all  wave-lengths,  but  is,  on  the  con- 
trary, a  discontinuous  one  in  which  vibrations  of  certain 
lengths  are  missing  from  the  sum-total  which  goes  to  make 
up  white  light. 

If  the  sun  is  remarkable  for  this  discontinuous  spectrum, 
what  types  of  spectra  do  the  other  heavenly  bodies  show? 
If  Fraunhofer  could  examine  the  light  from  a  small  round 
opening  at  a  short  distance  from  his  instrument,  there  is  no 
reason,  if  the  light  were  sufficient,  why  he  should  not  ex- 
amine the  light  from  a  round  body  made  apparently  small 
by  the  fact  of  its  situation  at  a  great  distance.  He  ex- 
amined, therefore,  the  light  from  Venus  directly  without 
making  the  light  pass  through  a  small  opening,  and  he  found, 
after  spreading  out  the  light  by  means  of  the  cylindrical 
lens;  that  the  same  lines  appeared  in  the  light  of  Venus  as 


THE    SPECTROSCOPE  81 

appear  in  sunlight.  Since,  however,  the  light  from  the  planet 
is  very  feeble  in  comparison  with  the  light  received  from  the 
sun,  the  intensity  of  the  violet  and  red  colors  of  its  spectrum 
are  very  weak,  and  on  this  account  even  the  stronger  lines 
in  both  these  colors  are  seen  with  difficulty,  though  in  the 
other  colors  they  are  very  easily  distinguished.  Fraunhofer 
was  able  to  see  D,  E,  b  and  F  perfectly  defined  and  he  even 
recognized  in  the  triplet  b  in  the  green,  two  lines,  one  weak 
and  one  strong,  although  he  was  unable  to  see  that  the 
stronger  of  these  two  lines  was  really  a  double  line.  This, 
of  course,  was  due  to  the  weakness  of  the  spectrum,  and  for 
this  same  reason  the  other  finer  lines  could  not  be  distin- 
guished satisfactorily.  By  measuring  the  arcs  DE  and  EF 
it  was  made  certain  that  the  light  from  Venus  contained,  as 
far  as  could  be  analyzed,  just  the  same  lines  in  its  spectrum 
as  did  the  light  of  the  sun. 

With  this  same  apparatus  observations  were  made  on  the 
light  of  some  fixed  stars  of  first  magnitude,  but  since  the 
light  of  these  stars  is  much  weaker  than  that  of  Venus,  it 
is  natural  that  the  brightness  of  the  spectrum  should  be 
much  less.  In  spite,  however,  of  this  comparative  lack  of 
brilliancy,  Fraunhofer  was  able  to  recognize  with  certainty 
in  the  spectrum  of  Sirius  three  broad  lines  which  appear  to 
have  no  connection  with  those  of  sunlight:  one  of  these  is 
in  the  red,  one  in  the  green,  and  the  other  in  the  blue.  In 
the  spectra  of  other  fixed  stars  of  the  first  magnitude,  lines 
were  actually  recognized  by  him,  and  it  seemed  certain  that 
these  spectra  though  very  faint  differed  amongst  themselves. 
The  observations  were  made  with  a  telescope  of  an  aperture 
of  about  one  inch,  so  that  it  was  impossible  for  the  distin- 
guished pioneer  to  do  more  than  to  point  out  the  way.  In 
such  a  manner  as  this  was  the  science  of  stellar  spectroscopy 
born. 

As  we  have  seen,  Fraunhofer  examined  the  stellar  spectra 
by  allowing  the  light  from  the  star  to  fall  directly  on  the 
prism  and  after  refraction  to  examine  this  light  by  the 
telescope.  This  same  method  of  observation  is  still  of  the 
greatest  scientific  value  and  it  is  in  this  way  that  Harvard 
College  Observatory  has  been  able  to  accomplish  such  an 


82  ECLIPSES    OF    THE    SUN 

enormous  amount  of  sound  research  by  methods  involving 
the  use  of  the  spectroscope.  The  combination  of  prism  and 
object  glass  is  called  an  "  objective  prism  "  —  or,  when  used 
for  photographing,  the  "  prismatic  camera."  If  we  point 
such  an  instrument  to  any  place  in  the  sky,  we  can  view  by 
our  eye,  or  photograph  on  a  plate,  the  spectrum  not  of  one 
star  only,  but  of  all  the  stars  that  are  in  the  field  of  the 
telescope. 

It  is  not  to  be  wondered  at  that  a  man  who  had  thus 
brought  sun,  planet  and  star  within  the  grasp  of  a  new  in- 
strument should  not  rest  content  with  these  observations. 

Fraunhofer  next  investigated  at  considerable  length  the 
spectra  of  artificial  light.  In  an  early  part  of  his  paper  he 
states  that,  on  examining  the  spectra  of  flames,  he  found 
that  flames  such  as  that  of  a  lamp  and  candle,  and,  indeed, 
in  general  the  light  produced  by  the  flame  of  a  fire,  exhibit 
between  the  red  and  yellow  of  the  spectrum  a  clear  and  well 
marked  line  which  occupies  the  same  place  in  all  the  spectra. 
Returning  to  this  subject  later,  he  notes  that  in  transmitting 
the  light  of  a  lamp  through  the  same  aperture  employed  for 
the  examination  of  the  solar  spectrum,  a  line  appears  which 
corresponds  exactly  to  the  position  of  the  D  line  in  the  solar 
spectrum.  In  fact,  the  resemblance  to  the  solar  D  line  is  so 
close  that  both  the  bright  artificial  line  and  the  solar  D  line 
are  each  a  fine  double  line.  This  was  the  first  step  towards 
the  true  explanation  of  the  dark  lines  in  the  solar  spectrum; 
but  it  took  many  years  before  the  true  meaning  was  arrived 
at,  mainly  on  account  of  the  presence  of  this  bright  D  pair 
in  practically  every  flame  and  under  all  conceivable  sets  of 
conditions. 

According  to  Agnes  M.  Clerke,1  "  the  ubiquity  and  con- 
spicuousness  of  the  sodium-line  long  impeded  progress.  It 
was  elicited  by  the  combustion  of  a  surprising  variety  of 
substances  —  sulphur,  alcohol,  ivory,  wood,  paper;  its  per- 
sistent visibility  suggesting  the  accomplishment  of  some  uni- 
versal process  of  nature  rather  than  the  presence  of  one  in- 
dividual kind  of  matter.  But  if  spectrum  analysis  was  to 
exist  as  a  science  at  all,  it  could  only  be  by  attaining  cer- 

1  History  of  Astronomy  during  the  Nineteenth  Century. 


THE    SPECTROSCOPE  83 

tainty  as  to  the  unvarying  association  of  one  special  sub- 
stance with  each  special  quality  of  light.  It  appeared,  in- 
deed, without  fail  where  sodium  was;  but  it  also  appeared 
where  it  might  be  thought  only  reasonable  to  conclude  that 
sodium  was  not.  Nor  was  it  until  thirty  years  later  that 
William  Swan,  by  pointing  out  the  extreme  delicacy  of  the 
spectral  test,  and  the  singularly  wide  dispersion  of  sodium, 
made  it  appear  probable  (but  even  then  only  probable)  that 
the  questionable  yellow  line  was  really  due  invariably  to 
that  substance.  Common  salt  (chloride  of  sodium)  is,  in 
fact,  the  most  diffusive  of  solids.  It  floats  in  the  air;  it  flows 
with  water;  every  grain  of  dust  has  its  attendant  particle; 
its  absolute  exclusion  approaches  the  impossible.  And 
writhal  the  light  that  it  gives  in  burning  is  so  intense  and 
concentrated,  that  if  a  single  grain  be  divided  into  180  mil- 
lion parts,  and  one  alone  of  such  inconceivably  minute  frag- 
ments be  present  in  a  source  of  light,  the  spectroscope  will 
show  unmistakably  its  characteristic  beam." 

The  advance  made  by  Fraunhofer  in  studying  the  re- 
fraction of  light  through  prisms  placed  the  new  science  on 
a  very  firm  foundation.  This  remarkable  progress,  due  to 
the  very  marked  improvement  in  the  quality  of  the  glass 
used  in  the  prisms,  was  rendered  possible  only  by  the  em- 
ployment of  a  telescope  of  greatly  increased  defining  power. 
The  utilization  of  the  telescope  permitted  advances  in  the 
study  of  diffraction  of  light  through  a  narrow  aperture  or 
slit,  as  important  and  as  epoch-making  as  the  investigations 
concerning  the  refraction  of  light.  The  instrument  used  by 
Fraunhofer  was  essentially  a  twelve-inch  repeating  theodo- 
lite whose  verniers  read  to  4".  In  the  middle  of  the  circle 
and  about  it  there  is  a  plane  horizontal  disk  six  inches  in 
diameter  which  turns  on  its  axis,  and  whose  center  lies  ex- 
actly on  the  axis  of  the  theodolite.  On  this  disk,  the  slit  to 
be  investigated  was  placed.  The  width  of  the  opening  in 
the  slit  was  measured  by  a  micrometer  devised  for  the  pur- 
pose which  could  read  to  o.oooi  inches.  Light  passing 
through  a  narrow  opening  at  the  heliostat  and  falling  on  the 
screen  with  its  slit  is  examined  by  the  theodolite  telescope. 
Fraunhofer  discovered  the  diffraction  pattern  which  consists 


84  ECLIPSES    OF    THE    SUN 

of  a  bright  strip  in  the  center  having  symmetrically  on  each 
side  a  series  of  bright  and  dark  bands,  if  the  incident  light 
is  monochromatic.  If  the  incident  light  is  sunlight,  instead 
of  having  bright  and  dark  bands,  there  will  be  found  a  series 
of  colored  bands  in  which,  however,  the  transitions  from  one 
color  to  another  are  not  sharply  defined.  The  series  of 
bands  gradually  decrease  in  brightness  in  passing  outwards 
from  the  central  beam  of  light  until  they  are  finally  rendered 
invisible.  Instead  of  employing  a  long,  narrow  opening  at 
the  heliostat,  he  used  a  small  circular  opening,  in  front  of 
which  he  placed  a  cylindrical  lens  and  as  a  result  obtained 
bands  identical  with  those  produced  by  the  other  method. 
In  the  process  of  development  of  this  subject,  the  natural 
course  after  trying  diffraction  through  a  single  opening, 
would  be  to  try  the  action  of  two,  three  and  more  parallel 
openings.  This  was  exactly  the  plan  followed  by  Fraun- 
hofer.  In  order  to  study  the  diffraction  through  a  great 
number  of  openings  he  stretched  upon  a  rectangular  frame 
a  great  many  wires  of  the  same  thickness,  parallel  to  each 
other  and  at  the  same  distance  apart.  The  light  was  then 
diffracted  through  the  intervening  spaces.  In  order  to  be 
sure  that  the  wires  were  exactly  parallel  and  at  exactly  equal 
distances  apart,  he  made  two  very  good  micrometer-screws, 
and  putting  these  on  opposite  sides  of  a  frame,  he  wound 
on  this  frame  very  fine  wire,  being  careful  to  stretch  the 
wire  at  a  constant  tension.  If  now  he  soldered  along  the 
length  of  the  screws,  each  wire  was  thus  securely  fastened, 
and  by  sawing  each  screw  in  halves,  two  similar  wire  grat- 
ings were  obtained.  By  this  method  gratings  were  manu- 
factured consisting  of  wires  0.002  inches  thick,  and  sepa- 
rated by  spaces  of  0.004  inches.  Using  a  grating  of  260 
turns  of  wire,  and  examining  the  light  which  first  passed 
through  a  narrow  opening  at  the  heliostat  and  then  fell  on 
the  grating  placed  in  front  of  the  theodolite  objective,  he 
found,  much  to  his  surprise,  phenomena  which  were  entirely 
different  from  those  observed  with  a  single  opening.  The 
aperture  at  the  heliostat  was  seen  exactly  as  if  no  grating 
intervened,  but  on  both  sides  were  seen  a  series  of  spectra 
as  perfect  as  he  had  hitherto  obtained  with  a  good  prism. 


SUN-SPOT  WHICH  DEVELOPED  IN  TWENTY-FOUR  HOURS 

Photographed  at  Mt.  Wilson.     The  black  circle  at  left  of  lower  picture 
shows  the  size  of  the  earth. 


W 

H 


H 

§ 

N 

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a 

o 


W 
0, 

O 

u 


o 
H 

§ 

o 


THE    SPECTROSCOPE  85 

The  series  of  spectra  gradually  increased  in  length  but  de- 
creased in  intensity,  and  with  his  apparatus  he  was  able  to 
count  thirteen  spectra  on  both  sides  of  the  middle.  In  order 
to  vary  the  conditions  as  much  as  possible,  gratings  were 
made  of  different  thickness  of  wire  and  with  different  spaces. 
Wire  was  wound  on  a  screw  having  as  many  as  343  threads 
to  the  inch.  Gratings  were  also  made  by  scratching  parallel 
lines  on  a  piece  of  glass  covered  with  goldfoil,  through  which 
spectra  were  observed  exactly  similar  to  those  observed  with 
wire  gratings.  Fraunhofer  quickly  found  that  the  size  of 
the  spectra  produced  did  not  depend  upon  the  width  of  the 
spaces  nor  upon  the  thickness  of  the  wires,  but  upon  the 
sum  of  these  two  quantities,  or  the  distance  apart  of  the 
centers  of  the  wires.  Consequently,  the  finer  the  screw  in 
whose  grooves  the  wires  were  stretched,  the  longer  would 
be  the  spectra,  and  it  became  immaterial  of  what  thickness 
the  wire  was  or  how  wide  the  opening.  The  quality  and 
accuracy  of  a  grating  depends  on  the  precision  attained  in 
the  attempt  to  arrange  wires  of  the  same  width  throughout, 
and  in  making  the  wires  perfectly  parallel,  with  their  centers 
equally  distant. 

Although  the  same  dark  lines  were  seen  in  the  spectrum 
of  sunlight  when  produced  by  the  grating  as  were  found 
when  a  prism  was  employed,  one  point  of  difference  was 
revealed  in  the  relative  distances  apart  of  the  lines  in  the 
two  spectra.  In  the  diffraction  spectrum  of  the  grating,  each 
of  the  different  colors,  the  red,  orange  and  so  on  through 
the  blue  and  violet,  is  about  equal  in  extent,  while  in  the 
prismatic  spectrum  on  the  other  hand  the  colors  at  the  red 
end  grow  more  and  more  crowded  together,  with  the  result 
that  the  violet  reaches  to  a  much  greater  extent  in  prismatic 
spectra  than  the  red.  Consequently,  the  appearance  of  a 
diffraction  spectrum  differs  very  much  from  that  of  a  pris- 
matic spectrum,  and  it  is  well  to  be  familiar  with  the  two 
different  types  of  spectra. 

In  a  normal  spectrum  produced  by  a  grating,  the  disper- 
sion from  the  C  line  in  the  red  to  the  D  lines  in  the  orange 
is  approximately  twice  the  distance  from  G  to  the  H-line  in 
the  violet;  whereas  with  a  flint  prism  of  27°  angle  the  ex- 


86  ECLIPSES    OP    THE    SUN 

tents  of  the  red  and  violet  are  changed  in  such  a  remarkable 
manner  that  the  distance  from  C  to  D  is  only  one-half  that 
from  G  to  H. 

There  is  also  another  marked  difference  between  prismatic 
and  diffraction  spectra.  In  using  a  prism  to  form  a  spec- 
trum, the  red  rays  are  the  least  bent  from  their  incident  di- 
rection, and  as  a  result  we  speak  of  the  red  as  the  least 
refrangible.  How  is  it  with  the  diffraction  spectrum  with  the 
bright  beam  in  the  center  and  spectra  on  both  sides?  Is  the 
violet  or  the  red  end  towards  the  bright  patch?  The  red  is 
the  least  refrangible  end  in  prismatic  spectra,  but  not  in 
diffraction  spectra,  for  the  violet  is  nearest  to  the  bright 
patch  with  the  red  end  bent  more  from  the  original  direc- 
tion. However,  since  spectrum  analysis  was  first  developed 
from  the  use  of  prisms,  a  nomenclature  has  been  adopted 
which,  though  applying  essentially  only  to  prismatic  spectra, 
is  used  indiscriminately  in  respect  to  spectra  of  both  kinds. 
Thus  in  referring  to  the  least  refrangible  rays,  the  red  end 
of  the  spectrum  is  always  meant  and  never  the  violet,  al- 
though in  the  diffraction  spectrum  the  violet  is  the  least 
refrangible  end. 

After  measuring  the  angles  by  means  of  the  theodolite, 
Fraunhofer  was  able  to  formulate  two  laws:  first,  the  size 
of  the  spectra,  and  their  distances  from  the  center  (or  the 
dispersion)  vary  inversely  as  the  distance  between  the  cen- 
ters of  the  lines  in  the  grating;  and  second,  the  dispersions 
in  the  different  spectra  for  any  ray  form  an  arithmetical 
series. 

This  second  law  states  in  other  words  that  the  angle  of 
deflection  of  the  same  colored  beams  in  the  series  of  spectra 
formed  by  the  grating  are  in  the  ratio  of  the  numbers  i,  2, 
3,  etc.  The  experiments  from  which  these  results  were  de- 
duced gave,  however,  such  small  angles  that  the  sine,  the 
tangent  and  the  arc  do  not  sensibly  differ.  If  the  angles 
were  larger,  that  is,  if  the  gratings  had  greater  dispersion, 
it  might  be  possible  to  determine  whether  it  was  the  arcs 
themselves  that  form  an  arithmetical  series,  or  some  func- 
tion of  these  angles.  Having  this  in  view,  Fraunhofer  made 
further  experiments  on  gratings  to  find  whether  it  would  not 


THE    SPECTROSCOPE  87 

be  possible  to  get  gratings  of  greater  dispersion.  As  it  was 
almost  impossible  to  evolve  a  screw,  for  the  manufacture  of 
his  wire  gratings,  with  a  smaller  pitch  than  the  one  he  had 
already  employed  of  343  threads  to  the  inch,  he  constructed 
a  machine  for  scratching  parallel  lines  upon  a  piece  of  plane 
glass  covered  with  gold-foil,  and  he  succeeded  in  scratching 
them  so  closely  together  that  he  was  able  to  rule  a  grating 
with  about  900  lines  per  inch.  If  more  lines  than  this  were 
scratched,  practically  no  gold- foil  remained.  With  this  grat- 
ing and  with  others,  he  measured  the  deflection  for  different 
rays  in  different  orders  of  spectra  and  he  found  that  the 
sines  of  the  angles  of  deflection  increased  uniformly  in  the 

different  orders  of  spectra,  or  in  other  words  Sin  0  =  — -, 

0} 

where  0  is  the  angle  of  deflection,  n  is  the  order  of  spectrum, 
X  is  the  wave-length  and  co  the  grating  space. 

Using  this  equation,  Fraunhofer  was  able  to  astonish  the 
world  by  telling  them  that  he  had  been  able  to  measure  the 
infinitesimal  length  of  a  light-wave,  for  the  D  line  of 
0.00005888  cm.,  which  is  very  close  to  modern  determi- 
nations. 

To  sum  up  the  work  of  Fraunhofer:  he  was  a  telescope 
maker,  and  in  order  to  improve  his  lenses  entered  into  an 
investigation  of  prisms  in  order  to  study  the  action  of  light 
in  passing  through  them.  Using  a  telescope  in  connection 
with  his  apparatus,  he  found  the  spectrum  of  sunlight  filled 
with  lines,  now  called  "  Fraunhofer  lines."  Investigating 
the  spectra  of  flames  he  found  the  bright  D  line  doubled  and 
in  the  same  position  exactly  as  the  dark  D  in  the  solar  spec- 
trum. He  examined  the  spectra  of  planets  and  stars.  Tak- 
ing up  diffraction,  he  studied  the  action  of  light  passing 
through  a  single  opening,  and  formulated  the  laws  govern- 
ing it.  He  then  studied  the  action  of  light  through  parallel 
openings  side  by  side.  Next  he  attempted  to  rule  lines  in 
a  layer  of  grease  spread  over  a  glass  plate  so  thinly  that  the 
film  could  scarcely  be  recognized  by  the  eye  alone.  In  this 
grease  parallel  lines  were  scratched  which  were  only  half  as 
far  apart  as  the  lines  ruled  in  gold-foil.  After  many  experi- 
ments it  was  found  impossible  to  rule  lines  in  any  layer  of 


88  ECLIPSES    OF    THE    SUN 

grease  or  varnish  much  finer  than  the  gold-foil  grating.  A 
diamond  point  is  the  only  method  yet  known  in  modern  en- 
gineering that  will  provide  sharp  and  sufficiently  clean  cut 
lines  to  permit  the  construction  of  finer  gratings. 

Let  us  stop  a  moment  and  think  what  it  means  to  rule  a 
grating  with  a  diamond  and  try  to  visualize  the  total  lengths 
of  the  lines  the  diamond  point  must  trace,  practically  with- 
out variation.  The  finest  gratings  ruled  by  Rowland  have 
20,000  lines  per  inch,  and  if  the  grating  possesses  a  ruled  sur- 
face of  3  x  6  inches,  the  diamond  evidently  rules  3x6 
x  20,000  or  360,000  inches,  a  distance  of  nearly  six 
miles.  If  in  ruling  this  distance  the  diamond  point 
wears  appreciably  or  breaks  down,  the  previous  rulings 
are  useless.  Altogether  it  takes  from  five  to  six  days 
continuous  working  to  rule  such  a  grating.  What  a  task 
Fraunhofer  must  have  had  in  ruling  his  gratings!  In- 
deed it  must  have  consumed  an  enormous  amount  of 
time  and  patience,  with  numerous  vexations  caused  by 
imperfect  rulings  and  the  fracture  of  the  diamond  points. 
It  was  only  after  many  trials  that  he  succeeded  finally  in 
getting  a  grating  with  about  7500  lines  to  the  inch.  In 
order  to  obtain  good  definition  with  his  little  telescope  it 
was  necessary  for  the  slit  to  be  at  some  considerable  dis- 
tance (as  much  as  642  feet  in  one  set  of  experiments)  in 
front  of  the  prism.  A  great  advance  in  instrumental  equip- 
ment was  later  made  by  Simms,  of  the  celebrated  optical  firm 
of  Troughton  and  Simms,  who  rendered,  by  the  introduction 
of  a  lens  between  slit  and  prism,  the  incident  rays  parallel 
in  falling  on  the  prism.  Thus  was  introduced  the  collimat- 
ing  lens  which  increased  the  compactness  of  the  spectroscope 
and  rendered  it  almost  as  we  now  use  it. 

In  England,  Brewster  and  Herschel  undertook  a  great  va- 
riety of  experiments  by  means  of  which  they  examined  the 
absorbing  effects  of  various  colored  substances.  A  piece  of 
red  glass  permits  only  the  red  part  of  the  spectrum  to  pass 
through  it,  the  balance  of  the  spectrum  being  absorbed  by 
the  glass,  while  blue  glass  allows  only  the  blue  end  of  the 
spectrum  to  pass  unobstructed.  It  consequently  became 
evident  that  the  absorbing  effect  of  the  terrestrial  atmos- 


THE    SPECTROSCOPE  89 

phere  should  be  carefully  investigated  before  any  certain 
information  could  be  obtained  regarding  the  cause  of  the 
dark  lines  in  the  solar  spectrum.  The  water  vapor  in  our 
atmosphere  exerts  a  powerful  absorbing  action  on  the  light 
of  the  sun  as  it  passes  through  the  air  on  its  way  to  reach 
the  slit  of  the  spectroscope.  The  absorption  commences  at 
the  blue  end  of  the  spectrum  and  becomes  gradually  greater 
and  greater  as  the  sun  sinks  towards  the  horizon.  For  very 
dense  layers  of  atmosphere,  when  the  sun  is  near  the  horizon 
either  in  rising  or  setting,  the  absorbent  action  is  s5  great 
that  little  of  the  solar  spectrum  penetrates  except  the  yellow 
and  red  parts,  the  violet  end  being  entirely  absorbed.  For 
this  reason  the  sun  is  always  red  when  rising  or  setting,  and 
the  color  becomes  a  deeper  red  when  dust  or  haze  in  the 
atmosphere  causes  a  greater  absorbing  effect.  At  the  time 
of  a  total  eclipse  of  the  moon,  the  light  from  the  sun  passes 
through  what  might  be  termed  a  double  layer  of  terrestrial 
atmosphere  with  the  consequence  that  there  is  an  increased 
absorption,  the  moon  thus  shining  with  a  dull  copper  colored 
hue,  and  this  in  spite  of  the  fact  that  the  moon  is  immersed 
in  the  shadow  cast  by  the  earth! 

By  1833,  Brewster  announced  that  he  had  examined  the 
lines  of  the  solar  spectrum  with  various  optical  combina- 
tions, and  had  made  a  map  of  the  solar  spectrum  on  a  scale 
four  times  that  of  Fraunhofer.  Indeed  for  some  portions 
of  the  spectrum  the  scale  was  twelve  times  Fraunhofer's. 
By  observing  the  absorbing  effect  of  various  substances  he 
soon  found  that  some  of  these  substances  exerted  a  general 
darkening  action  on  the  spectrum,  whereas  other  materials 
produced  an  absorption  in  a  limited  portion  of  the  spectrum. 
At  times  the  effect  was  so  limited  in  action  that  bands  or 
even  lines,  sharp  and  distinct,  were  added  to  the  solar  spec- 
trum. These  phenomena  were  so  clearly  defined  that  the 
conviction  was  borne  in  upon  Brewster  that  he  had  obtained 
"  the  discovery  of  a  general  principle  of  chemical  analysis, 
in  which  simple  or  compound  bodies  might  be  characterized 
by  their  action  on  definite  parts  of  the  spectrum.'7 

In  the  course  of  his  investigations,  Fraunhofer  varied  the 
conditions  under  which  he  observed,  using  different  kinds 


90  ECLIPSES    OF    THE    SUN 

of  slits  to  see  if  the  dark  lines  in  the  solar  spectrum  were 
caused  by  some  action  at  the  slit  itself;  but  when  he  found 
that  the  form  of  the  slit  had  nothing  to  do  with  the  presence 
of  the  lines,  he  concluded  that  the  lines  must  be  truly  solar 
in  their  origin.  Brewster  endorsed  this  view  and  inferred 
that  if  a  tube  of  nitrous  oxide  gas  gave  lines  identical  in 
character  with  the  solar  lines,  these  lines  must  then  be 
caused  by  absorption  at  the  surface  of  the  sun.  When  in 
addition  he  found  that  many  of  the  lines  of  nitrous  acid  gas 
appeared  to  be  identical  in  position  with  some  of  the  Fraun- 
hofer  lines,  the  verification  seemed  to  be  complete.  Brewster 
made  another  important  discovery  which  is  described  in 
his  own  words  as  follows :  "  When  the  sun  descends  towards 
the  horizon  and  shines  through  a  rapidly  increasing  depth  of 
air,  certain  lines  which  before  were  little,  if  at  all,  visible, 
become  black  and  well  defined,  and  dark  lines  appear  even 
in  what  were  formerly  the  most  luminous  parts  of  the  spec- 
trum." As  these  lines  appear  both  at  sunrise  and  sunset, 
Brewster  announced  the  discovery  that  these  bands  and  lines 
were  caused  by  the  absorbent  effect  of  the  earth's  atmos- 
phere. Since  the  majority  of  the  lines  in  the  sun,  however, 
appeared  without  change,  he  concluded  that,  "  the  apparent 
body  of  the  sun  is  not  a  flame  in  the  ordinary  sense  of  the 
word,  but  a  solid  body  or  coating  raised  by  intense  heat  to  a 
state  of  brilliant  incandescence." 

To  quote  from  Lockyer,  Chemistry  of  the  Sun,  page  41, 
"  It  will  be  seen,  then,  that  the  study  of  the  sun  was  now 
(1833)  in  full  swing.  We  had  at  length,  after  waiting  some 
centuries,  a  method  of  observing  a  spectrum;  we  had, 
further,  the  fact  that  there  were  dark  lines  in  the  solar 
spectrum;  that  colored  flames  gave  us  bright  lines;  that 
certain  substances  stopped  some  of  the  light  which  passed 
through  them,  thus  producing  dark  lines.  Hence  that  the 
solar  lines  might  be  produced  in  the  same  way." 


CHAPTER   VI 

THE    SPECTROSCOPE    (CONTINUED) 

THE  solar  eclipse  of  1836  played  a  very  important 
role  in  the  history  of  the  development  of  the  new 
science  of  astrophysics.  The  eclipse  was  visible  at 
Edinburgh.  It  was  not  a  total  eclipse,  however,  but  merely 
an  annular  one.  At  such  an  eclipse  the  angular  diameter  of 
the  moon  is  smaller  than  that  of  the  sun,  with  the  result  that 
at  the  middle  of  the  eclipse  there  is  a  ring  or  annulus  of  sun- 
light visible  around  the  edge  of  the  dark  moon.  This  eclipse 
was  observed  by  Forbes.  Referring  to  Brewster's  discovery 
of  atmospheric  lines  in  the  solar  spectrum,  Forbes  clearly 
pointed  out  that  the  "  telluric  "  lines  were  comparatively 
few  in  number  and  relatively  unimportant  compared  with 
the  very  great  number  of  solar  lines.  Moreover,  he  empha- 
sized the  fact  that  the  Fraunhofer  lines  could  not  have  been 
caused  by  the  absorbing  effect  in  our  terrestrial  atmosphere, 
for  if  this  were  true  the  spectra  of  the  stars  should  be  iden- 
tical with  that  of  the  sun.  As  the  spectra  of  the  stars  dif- 
fered among  themselves  and  were  not  always  identical  with 
the  spectrum  of  the  sun,  it  was  manifest  that  it  was  not  pos- 
sible to  imagine  a  terrestrial  origin  for  all  of  the  solar  lines. 
Consequently,  if  the  absorbing  action  of  the  earth's  atmos- 
phere could  not  be  invoked  to  give  an  adequate  explanation, 
there  remained  no  other  cause  than  that  the  origin  of  the 
lines  must  take  place  within  the  sun's  own  atmosphere.  If, 
therefore,  this  was  the  true  explanation,  then  an  annular 
eclipse  of  the  sun  should  furnish  a  crucial  test.  The  ter- 
restrial lines  of  the  solar  spectrum  become  more  and  more 
intensified  as  the  sun's  light  reaches  us  through  greater  and 
greater  layers  of  our  earth's  atmosphere,  and  in  like  fashion 
it  appeared  evident  to  Forbes  that  a  similar  effect  must  be 
visible  in  the  lines  of  truly  solar  origin.  And  since  the  light 

91 


92  ECLIPSES    OF    THE    SUN 

from  the  limb  of  the  sun  must  pass  through  a  much  greater 
thickness  of  solar  atmosphere  than  that  from  the  sun's  cen- 
ter, the  lines  from  the  sun's  edge,  observable  at  the  time  of 
the  annular  eclipse,  should  be  much  intensified  in  comparison 
with  the  Fraunhofer  lines  ordinarily  visible  from  the  sun's 
center.  When  the  eclipse  took  place  no  change  whatever 
was  observed  by  Forbes  in  the  number,  position  or  intensity 
of  the  lines,  and  the  obvious  conclusion  was  drawn  by  him 
that,  "  This  result  proves  conclusively  that  the  sun's  atmos- 
phere has  nothing  to  do  with  the  production  of  this  singular 
phenomenon." 

To  us  living  in  the  twentieth  century,  it  seems  passing 
strange  that  Forbes  did  not  try  the  experiment  of  comparing 
the  limb  and  the  center  of  the  sun  by  forming  an  image  of 
the  sun  on  the  slit  of  his  spectroscope  by  means  of  a  project- 
ing lens.  This  plan  was  undoubtedly  in  his  mind  as  the 
following  shows:  "Had  the  weather  proved  unfavorable 
for  viewing  the  eclipse,  I  intended  to  have  tried  the  experi- 
ment by  forming  an  image  of  the  sun  by  using  a  lens  of  long 
focus,  stopping  alternately  by  means  of  a  screen  the  interior 
and  central  moiety  of  his  rays,  and  restoring  the  remainder 
to  parallelism  by  means  of  a  second  lens,  then  suffering  these 
to  fall  on  the  slit  as  before.  The  result  of  my  experiment 
during  the  eclipse  seemed,  however,  so  decisive  as  to  no 
marked  change  being  produced  at  the  sun's  edges  that  I 
have  thought  it  unnecessary  to  repeat  it." 

The  scientific  world  in  the  first  half  of  the  nineteenth 
century  knew  little  of  laboratory  methods  and  it  was  not 
at  that  time  a  habit  of  mind  to  test  any  theoretical  conclu- 
sions by  means  of  experimentation.  In  fact  it  was  not  until 
the  year  1866  that  the  idea  of  Forbes  was  carried  into  execu- 
tion and  a  projecting  lens  utilized  for  the  examination  of 
local  phenomena  such  as  the  spectra  of  the  limb  or  of  sun 
spots. 

In  1845,  there  was  performed  in  England  by  W.  H.  Miller, 
the  very  experiment  that  later  in  Kirchhoff  s  hands  was  the 
crucial  proof  of  the  cause  of  the  Fraunhofer  lines.  This 
experiment  consisted  in  passing  sunlight  through  various 
vapors  heated  to  incandescence  in  a  flame  and  noting  the 


THE    SPECTROSCOPE  93 

changes  in  the  solar  spectrum.  He  observed  that  in  passing 
sunlight  through  glowing  sodium  vapor,  the  D  lines  were 
intensified.  It  is  surprising  that  he  appears  to  have  sought 
no  explanation  of  this  amazing  fact,  and  the  surprise  is  all 
the  greater  since  Miller's  experiments  were  carried  out  with 
the  express  purpose  of  testing  the  theory  that  the  Fraunhof  er 
lines  were  actually  produced  by  absorption  in  the  sun's 
atmosphere. 

This  line  of  investigation  was  continued  in  1849  by 
Foucault  in  Paris  who  was  able  to  use  a  new  method  of 
heating  salts  and  metals  to  the  glowing  point  by  the  use  of 
the  electric  arc.  He  focused  an  image  of  the  sun  on  the 
arc  itself  and  this  procedure  allowed  him  to  observe  at  the 
same  time  the  spectra  of  the  arc  and  of  sunlight.  Inciden- 
tally he  was  surprised  to  discover  the  extreme  transparency 
of  the  arc  which  caused  only  a  faint  shadow  in  the  sunlight. 
This  experiment  manifested  to  Foucault  that  when  the  two 
spectra  were  exactly  superimposed,  the  D  line  of  sunlight 
was  made  considerably  darker,  proving  that  the  arc  ab- 
sorbed the  D  rays;  but  that  when,  on  the  contrary,  the  two 
spectra  jutted  out  one  beyond  the  other,  the  D  line  appeared 
darker  than  usual  in  sunlight,  yet  stood  out  bright  in  the 
electric  spectrum,  thus  demonstrating  the  perfect  coinci- 
dence in  position  ot  the  dark  and  bright  rays.  "  Thus  the 
arc  presents  us  with  a  medium  which  emits  the  rays  D  on 
its  own  account,  and  which  at  the  same  time  absorbs  them 
when  they  come  from  another  quarter."  It  was  difficult  to 
explain  how  it  was  possible  for  a  glowing  flame  to  furnish 
at  the  same  time  both  bright  and  dark  lines,  —  and  the  riddle 
of  the  solar  lines  seemed  a  hard  one  to  solve.  As  early  as 
1850,  Stokes  seemed  to  have  clearly  grasped  the  solution 
of  the  problem,  and  he  inserted  a  discussion  of  these  matters 
in  his  university  lectures  at  Cambridge.1  Moreover,  he 
seems  to  have  been  the  first  to  localize  the  cause  of  the 
yellow  D  lines  since  he  observed  that  the  bright  line  was 
absent  from  a  candle  flame  when  the  wick  was  snuffed  clean, 
and  from  an  alcohol  flame  when  the  spirit  was  burned  in  a 
watch-glass.  Although  he  so  clearly  saw  that  the  D  lines, 

1  See  Lockyer,  Chemistry  of  the  Sun,  p.  51. 


94  ECLIPSES    OF    THE    SUN 

whether  bright  or  dark,  were  caused  by  sodium  vapor,  and 
therefore  correctly  concluded  that  the  absorbing  effect  of 
the  sodium  was  in  the  neighborhood  of  the  sun,  he  made  no 
further  experiments  to  test  his  deductions;  and  the  honor 
for  the  great  discovery  waited  another  nine  years.  Thus 
again  was  demonstrated  —  which  has  frequently  happened 
in  the  history  of  science  —  that  though  many  investigators 
have  converged  towards  the  same  goal  of  discovery,  yet  the 
prize  has  awaited  the  fortunate  one  who  should  make  the 
critical  experiment,  which  in  itself  at  times  has  been  one  of 
little  difficulty.  Thus  in  1855  Angstrom,  and  in  1859  Bal- 
four  Stewart  by  their  experiments  came  very  close  to  the 
true  solution  of  the  problem. 

In  1859,  Kirchhoff  showed  ]  for  the  first  time  that  in  order 
that  sodium  should  be  in  a  condition  to  absorb  from  Uie 
light  of  other  sources  it  must  itself  be  at  a  cooler  tempera- 
ture. "  Fraunhofer  has  remarked  that  in  the  spectrum  of  a 
candle  flame  two  bright  lines  appear  which  coincide  with  the 
two  dark  lines  D  of  the  solar  spectrum.  These  bright  lines 
can  be  easily  intensified  in  a  flame  into  which  some  common 
salt  is  put.  I  formed  a  solar  spectrum  by  projection  and  I 
allowed  the  solar  rays  thus  formed  to  pass  through  a  strong 
salt  flame  before  falling  on  the  slit.  If  the  sunlight  were 
sufficiently  subdued,  then  in  place  of  the  two  dark  lines  D 
two  bright  lines  appeared ;  if  the  intensity  increased  beyond 
a  certain  amount  then  the  two  dark  D  lines  showed  in  much 
greater  intensity  than  without  the  presence  of  the  salt  flame. 
The  spectrum  of  the  Drummond  light  contains  as  a  rule  the 
two  bright  sodium  lines  if  the  illuminating  spot  of  the  cal- 
cium cylinder  has  not  long  since  passed  the  glowing  point; 
if  the  cylinder  remains  undisturbed  then  these  lines  become 
weaker  and  finally  completely  vanish.  If  they  have  dis- 
appeared or  are  faintly  visible,  an  alcohol  flame  into  which 
cooking  salt  has  been  placed  and  which  is  brought  between 
the  calcium  cylinder  and  the  slit,  causes  two  dark  lines  of 
exceptional  blackness  and  sharpness,  which  in  that  respect 
agree  with  the  lines  D  of  the  solar  spectrum,  to  show  them- 
selves in  their  place.  In  this  manner  the  D  lines  of  the  solar 

1  For  complete  details  see  Kayser,  Handbuch  der  Spectroscopie,  Vol.  T,  p.  81, 


THE    SPECTROSCOPE  95 

spectrum  are  artificially  produced  in  a  spectrum  in  which 
they  are  naturally  not  present.  I  conclude  from  these  ob- 
servations that  colored  flames,  in  the  spectra  of  which  bright 
sharp  lines  are  found,  so  weaken  rays  of  the  color  of  these 
lines  when  such  rays  pass  through  the  flames,  that  in  place 
of  the  bright  lines,  dark  ones  appear  just  as  soon  as  there  is 
brought  behind  the  flame  a  source  of  light  of  sufficient  in- 
tensity in  the  spectrum  of  which  these  lines  are  otherwise 
lacking.  I  conclude  further  that  the  dark  lines  of  the  solar 
spectrum,  which  do  not  find  their  origin  in  the  earth's  atmos- 
phere, are  caused  in  the  glowing  solar  atmosphere  by  the 
action  of  those  substances  which  in  the  spectrum  of  a  flame 
produced  bright  lines  at  the  same  place.  We  thus  assume 
that  the  bright  lines  coinciding  with  D  in  the  spectrum  of 
a  flame  always  arise  from  sodium  contained  in  it;  the  dark 
D  lines  in  the  solar  spectrum  therefore  allows  us  to  conclude 
that  sodium  is  found  in  the  atmosphere  of  the  sun.  ...  In 
order  that  the  D  lines  should  come  out  dark  in  the  spectrum 
of  the  Drummond  light  it  is  necessary  to  use  a  salt  flame 
of  lower  temperature." 

In  these  experiments,  the  salt  flame  was  kept  constant 
the  intensity  of  the  sunlight  being  weakened  or  strength- 
ened at  leisure.  1$  other  words,  this  may  be  expressed  by 
saying  that  the  Fraunhofer  lines  in  the  spectrum  of  the  sun 
are  dark  only  in  contrast  with  the  more  brilliant  background 
of  the  sun  itself.  If  this  dazzling  surface  could  be  removed, 
and  the  comparatively  dark  lines  of  the  solar  spectrum  could 
then  be  viewed  against  a  background  still  darker,  then  by 
contrast  the  spectrum  lines  would  appear  as  bright  lines  on 
a  dark  background  where  formerly  they  had  existed  as  dark 
lines  on  a  bright  background.  This  change  in  the  sun's 
spectrum,  as  we  shall  see  later,  takes  place  at  the  time  of 
a  total  eclipse  of  the  sun.  The  student  of  spectroscopy  will 
save  himself  needless  worry  if  he  will  remember  that  bright 
and  dark  are  always  to  be  considered  as  relative  terms  only. 

A  spectrum  of  bright  lines  on  a  dark  background  is  said 
to  be  a  bright-line,  or  an  emission  spectrum.  On  the  other 
hand,  a  spectrum  of  dark  lines  on  a  bright  background  is 
called  a  dark-line,  or  an  absorption  spectrum.  If  a  chemi- 


96  ECLIPSES    OF    THE    SUN 

cal  element  is  heated  to  the  point  of  vaporization,  its  spec- 
trum consists  of  a  bright-line  spectrum.  Sodium  gives  a 
very  simple  spectrum,  consisting  mainly  of  two  very  strong 
lines  in  the  yellow  part  of  the  spectrum,  the  well-known  D 
lines.  If  pure  metallic  sodium  alone  is  used,  the  spectrum 
consists  of  these  D  lines.  If  the  sodium  is  in  chemical  com- 
bination with  chlorine,  and  the  sodium  chloride,  or  common 
salt,  is  heated  to  incandescence,  the  same  D-lines  due  to 
sodium  are  shown  in  the  bright-line  spectrum.  Or  if  any 
other  compound  of  sodium  is  heated,  the  same  D-lines  re- 
sult. The  manner  of  the  heating  is  of  no  consequence;  a 
pinch  of  common  cooking  salt  may  be  placed  on  the  wick  of 
an  alcohol  flame,  a  paper  soaked  in  a  saline  solution  may  be 
placed  about  the  burner  of  a  Bunsen  lamp,  or  a  grain  of 
salt  may  be  put  on  the  carbon  of  the  electric  arc  or  on  one 
of  the  poles  of  an  electric  spark  —  the  lines  of  sodium  will 
always  appear,  the  color  of  the  lines  will  be  exactly  the 
same,  and  the  wave-lengths  of  the  lines  will  be  unaltered 
no  matter  what  the  chemical  compound  in  which  the  sodium 
is  found  or  the  manner  of  heating  that  salt  to  incandescence. 
Since  the  total  light  of  the  sodium  consists  mainly  of  two 
lines  in  the  yellow,  then  when  sodium  is  burning,,  it  will 
give  off  yellow  light  only  and  the  color  of  the  flame  will 
appear  yellow  to  the  eye. 

If  another  element  like  lithium  is  examined,  whether  pure 
lithium  or  lithium  in  compound  with  some  other  element  or 
elements,  it  will  give  its  own  peculiar  spectrum  of  bright 
lines,  and  these  bright  lines  will  be  found  not  to  coincide 
with  the  D  lines  due  to  sodium.  Since  lithium  burns  with  a 
red  light,  the  prominent  lines  in  its  spectrum  will  be  found 
at  the  red  end  of  the  spectrum.  Some  metals  like  sodium 
show  a  very  simple  spectrum,  with  very  few  lines;  other 
metals  show  more  lines,  the  greatest  number  of  lines  appear- 
ing for  any  one  element  being  due  to  the  presence  of  iron. 
It  makes  no  difference  how  the  iron  is  heated  to  incandes- 
cence, it  makes  no  difference  whether  the  iron  is  a  piece  of 
scrap  or  of  polished  steel,  the  spectrum  will  consist  of 
thousands  of  bright  lines  in  all  the  colors  of  the  spectrum. 
Each  of  these  many  thousands  of  lines  has  its  own  particular 


THE    SPECTROSCOPE  97 

wave-length.  It  is  the  business  of  the  spectroscopist  to  find 
the  value  of  the  wave-length  of  each  and  every  line.  Some 
of  the  lines  are  faint,  some  strong,  some  are  narrow,  some 
broader,  some  are  very  sharp,  others  more  fuzzy  in  appear- 
ance, but  no  matter  what  the  quality  of  the  iron  or  how 
vaporized,  the  spectrum  is  the  same  with  lines  of  practically 
identical  wave-lengths.  Although  there  exist  a  great  many 
chemical  elements,  the  spectrum  of  each  of  which  consists 
of  many  lines,  while  others  of  the  elements  have  even  thou- 
sands of  lines  in  their  spectra,  it  may  almost  be  said  that 
no  line  in  any  one  spectrum  coincides  precisely  with  a  line 
in  any  other  spectrum.  If,  therefore,  it  is  possible  to  de- 
termine the  exact  wave-length  of  a  line  in  a  spectrum, 
though  the  chemical  origin  of  this  line  may  be  unknown, 
we  shall  have  a  ready  means  of  identifying  the  elemental 
source  of  this  line. 

The  principles  upon  which  spectrum  analysis  depends  are 
found  in  Young's  General  Astronomy,  page  213,  as  follows: 

1.  A  continuous  spectrum  is  given  by  every  incandescent 
body,  the  molecules  of  which  so  interfere  with  each  other 
as  to  prevent  their  free,  independent,  luminous  vibration; 
that  is,  by  bodies  which  are  either  solid  or  liquid,  or  if  gase- 
ous, are  under  high  pressure. 

2.  The  spectrum  of  a  gaseous  element,  under  low  pressure, 
is  discontinuous,  or  in  other  words  made  up  of  bright  lines, 
these  lines  being  characteristic,  that  is,  the  same  substance 
under  similar  conditions  always  gives  the  same  set  of  lines, 
and  generally  does  so  even  under  widely  different  conditions. 

3.  A  gaseous  substance  absorbs  from  white  light  passing 
through  it  precisely  those  rays  of  which  its  own  spectrum 
consists.    The  spectrum  of  white  light  which  has  been  trans- 
mitted through  it  then  exhibits  a  "  reversed  "  spectrum  of 
the  gas;  that  is,  one  which  shows  dark  lines  instead  of  the 
characteristic  bright  lines. 

The  third  law,  the  great  discovery  of  Kirchhoff,  may  be 
stated  in  other  words  as  follows:  The  relation  between  the 
emissive  power  for  each  wave-length  and  the  absorptive 
power  for  the  same  wave-length  at  the  same  temperature  is 
identical  for  all  bodies,  and  is  in  fact  equal  to  the  emissive 


98  ECLIPSES    OF    THE    SUN 

power  of  an  absolutely  black  body  at  the  same  wave-length 
and  temperature.  We  are  quite  familiar  with  a  similar  effect 
in  the  realm  of  sound.  If  a  voice  singing  or  speaking  sounds 
a  note  of  a  certain  pitch  in  a  room  where  there  is  a  piano, 
one  string  of  the  piano  will  vibrate  in  unison  with  the  voice, 
the  particular  piano  string  taking  its  motion  from  the  oscil- 
lations of  the  air.  Similarly,  a  tuning  fork  in  vibration  will 
set  in  motion  another  tuning  fork  nearby  which  is  tuned  to 
the  same  pitch. 

The  work  of  Kirchhoff,  therefore,  in  connecting  the  emis- 
sion of  light  with  absorption  gives  the  means  of  determining 
the  chemical  composition  of  the  sun.  According  to  our 
present  ideas,  the  photosphere  of  the  sun,  the  portion  of  the 
sun  we  see,  consists  of  gases  under  such  very  high  pressure 
that  the  molecules  cannot  vibrate  independently;  and  in 
consequence  the  spectrum  of  the  photosphere  must  be  con- 
tinuous, a  ribbon  of  light  without  breaks  from  red  to  violet. 
The  photosphere  is  surrounded  by  a  cooler  layer  of  gases 
under  low  pressure,  the  so-called  "  reversing  layer."  If  the 
spectrum  of  these  gases  could  be  examined  entirely  sepa- 
rated from  the  bright  photospheric  background,  they  would 
exhibit  the  gaseous,  or  bright-line  spectrum.  Under  ordi- 
nary conditions,  the  light  from  the  photosphere  shines 
through  the  cooler  gases  of  the  reversing  layer,  and  certain 
wave-lengths  are  there  absorbed  by  the  gases  of  the  revers- 
ing layer,  so  that  the  spectrum  of  the  sun  comes  to  us  as  a 
reversed  spectrum,  of  dark  lines  on  a  bright  background; 
these  dark  lines,  however,  as  stated  above,  are  dark  only  in 
contrast  with  the  much  brighter  photospheric  background. 
To  determine  the  constitution  of  the  sun,  it  becomes  there- 
fore necessary  to  compare  the  bright  line  spectra  of  the  va- 
rious elements  with  the  dark  line  spectrum  of  the  sun.  This 
comparison  may  be  made  by  two  different  methods,  either 
by  viewing  or  photographing  with  a  suitable  instrument  the 
spectrum  of  the  sun  and  the  comparison  spectrum  side  by 
side,  or  by  an  exact  determination  of  wave-lengths  in  the 
solar  and  in  the  comparison  spectrum. 

But  the  spectrum  of  the  sun  consists  of  many  thousands 
of  lines.     It  is  evidently  quite  possible,  and  even  highly 


THE    SPECTROSCOPE  99 

probable,  that  there  should  be  very  close  agreement  be- 
tween some  of  the  many  lines  in  the  sun  and  an  equal  num- 
ber of  lines  in  the  spectrum  of  the  element  under  investiga- 
tion. These  concurrences  might  be  the  result  of  pure  acci- 
dent. Kirchhoff  investigated  this  possibility.  A  particular 
line  in  a  comparison  spectrum  may  exactly  match  in  position 
a  line  in  the  solar  spectrum.  If  the  agreement  is  due  en- 
tirely to  chance,  then  by  the  laws  of  probability,  it  is  equally 
probable  that  the  line  in  the  spectrum  under  consideration 
may  or  may  not  match  a  line  in  the  sun's  spectrum,  or  speak- 
ing mathematically  the  chance  of  an  exact  match  taking 
place  fortuitously  is  one  out  of  two.  If  two  lines  agree  in 
each  spectrum,  then  the  possibility  of  this  happening  by 
chance  is  but  one  out  of  four.  Kirchhoff  found  sixty  lines 
of  iron  to  agree  with  sixty  lines  in  the  sun.  The  chance  that 
this  coincidence  of  all  sixty  lines  is  purely  accidental  is  ex- 
pressed by  the  number  ^  raised  to  the  sixtieth  power.  At 
the  present  time  over  two  thousand  lines  due  to  iron  have 
been  identified  in  the  solar  spectrum.  If  the  coincidence  of 
this  large  number  of  lines  were  the  result  of  pure  accident 
it  would  represent  a  chance  of  one  in  2  raised  to  the  2Oooth 
power.  This  number  is  about  equivalent  to  100  followed  by 
no  less  than  600  ciphers!  (If  one  has  nothing  better  to  do, 
one  might  take  a  large  piece  of  paper  and  put  down  the  num- 
ber one  and  follow  it  by  six  hundred  and  two  zeros.  Then 
one  could  divide  it  off  into  millions,  billions,  trillions,  etc., 
and  invent  a  name  for  this  huge  number ! )  The  chance  that 
the  lines  of  iron  and  the  Fraunhofer  lines  in  the  sun  should 
agree  in  position  entirely  by  accident  is  therefore  infinitesi- 
mally  small.  But  when  in  addition,  we  compare  the  appear- 
ance of  the  lines  in  the  two  spectra,  and  find  that  a  strong 
line  in  the  spectrum  of  the  sun  is  matched  by  a  strong  line 
in  the  spectrum  of  iron,  and  a  weak  Fraunhofer  line  is 
matched  by  a  weak  iron  line,  then  we  see  the  utter  impossi- 
bility of  the  coincidences  being  the  result  of  mere  chance. 
What  is  true  of  iron  is  equally  true  of  the  other  elements 
investigated.  It  accordingly  seems  perfectly  certain  that 
we  are  able  to  ascertain  the  chemical  constitution  of  the  sun 
by  means  of  the  spectroscope  even  though  we  are  looking 


100  ECLIPSES    OF    THE    SUN 

at  the  sun  across  a  space  of  ninety-three  millions  of 
miles. 

Astrophysics  thus  being  placed  on  a  very  firm  foundation, 
the  infant  science  was  immediately  recognized  throughout 
the  scientific  world  to  be  of  the  very  greatest  importance  to 
physicists,  chemists  and  astronomers.  But  in  spite  of  the 
almost  universal  recognition,  there  were  a  few  doubting 
Thomases.  In  Chambers's  very  excellent  Descriptive  As- 
tronomy, edition  of  1867,  page  27,  is  found  the  following: 
"  Spectrum  analysis  has  taken  a  start  within  the  last  two  or 
three  years,  chiefly  owing  to  the  assertions  made  that  it 
enables  us  to  ascertain  something  about  the  physical  condi- 
tion of  the  sun.  The  subject  is  too  purely  a  physical  one, 
and  also  in  too  infantine  a  state  to  require  notices  in  these 
pages  at  present,  though  the  time  may  come/' 

It  is  remarkable  to  read  the  early  history  of  spectroscopy 
and  learn  of  the  great  opposition  of  certain  scientists  to 
the  acceptance  of  Kirchhoff's  proof;  and  it  was  but 
natural  that  there  should  be  many  claims  to  priority. 
By  1867,  however,  Pritchard  of  Oxford  summarized  the 
general  feeling  of  the  scientific  world  in  the  following  words : 
"  It  may  safely  be  asserted  of  Foucault  in  1849,  of  Stokes  in 
1850,  of  Angstrom  in  1855,  and  of  Balfour  Stewart  in  1859, 
that  each  of  them  was  in  possession  of  an  enunciated  truth, 
which,  had  they  traced  to  their  natural  and  inevitable  con- 
sequences, must  have  led  to  that  grand  generalization  which 
will  immortalize  the  name  of  Kirchhoff,  and  which  forms 
one  of  the  happiest  and  most  remarkable  discoveries  of 
modern  times/7 

With  the  trail  so  surely  blazed,  the  path  pointed  out  the 
direction  of  the  researches  to  be  taken  by  Kirchhoff's  suc- 
cessors. The  quest  had  a  two-fold  interest,  for  not  only 
did  the  new  method  serve  as  an  infallible  chemical  test  of 
terrestrial  substances,  but  it  gave  also  a  ready  means  of 
determining  the  constitution  of  the  sun  and  also  of  the  more 
distant  suns,  the  stars.  Physicists,  astronomers  and  chem- 
ists vied  with  each  other  in  pushing  forward  the  researches 
as  rapidly  and  thoroughly  as  possible,  and  opticians  and 
instrument  makers  came  to  the  assistance  of  the  scientists 


THE    SPECTROSCOPE  101 

by  furnishing  improved  forms  of  apparatus.  It  was  mani- 
festly necessary  to  investigate  the  solar  spectrum  and  the 
spectra  of  the  various  chemical  elements.  These  investiga- 
tions had  necessarily  to  be  carried  out  with  the  spectra, 
produced  on  as  large  a  scale  as  possible,  and  the  measure- 
ments of  the  positions  of  the  lines  required  the  very  highest 
degree  of  precision  attainable.  To  denote  the  position  of  a 
line,  some  more  accurate  method  was  necessary  than  that  of 
placing  it  in  the  red  or  blue  of  the  spectrum.  Fraunhofer 
and  also  Kirchhoff  used  a  rather  arbitrary  scale.  Newton 
proved  that  a  difference  in  color  meant  a  difference  in  re- 
frangibility.  Fraunhofer  went  a  step  further  and  demon- 
strated that  a  difference  in  color  meant  a  difference  in  the 
length  of  the  wave  causing  the  light.  Since  the  time  of 
Thomas  Young,  1802,  it  has  been  known  that  light  is  a 
wave  phenomenon,  the  waves,  somewhat  similar  to  water 
waves,  moving  transversely  to  the  direction  of  motion. 
The  length  of  the  wave  from  crest  to  crest,  or  from  trough 
to  trough,  is  known  as  the  wave-length,  and  this  length 
might  be  measured  in  fractions  of  an  inch,  or  foot,  or  meter. 
All  scientists,  whether  they  live  in  America  or  in  Germany, 
or  whether  they  speak  English,  Japanese  or  Russian,  now 
use  the  meter  as  the  unit  for  measuring  the  wave-length 
of  light.  A  meter  divided  into  ten  thousand  million  parts, 
or  10  10  parts,  is  called  a  "  tenth-meter."  This  very  small 
distance  is  known  as  the  "  Angstrom  Unit,"  or  more  simply 
as  the  "  Angstrom,"  and  it  is  the  unit  for  measuring  wave- 
lengths. The  position  of  a  line  is  known  by  its  wave-length, 
and  the  more  precise  the  investigation  the  more  accurately 
do  we  need  to  know  this  quantity.  The  K-line  in  the  solar 
spectrum  has  a  wave-length  according  to  Rowland  of  3933.- 
826.  This  is  printed  either  as  X  3933.826,  or  3933.826  A. 
(We  shall  adopt  the  latter  notation.) 

Knowing  the  wave-length  of  light  of  a  certain  color,  it 
is  a  very  simple  matter  to  calculate  the  number  of  waves 
that  enter  into  the  eye  in  a  single  second  of  time.  All  that 
is  required  is  to  know  the  velocity  at  which  light  travels. 
It  is  now  known  that  light  of  all  colors,  whether  red,  blue  or 
violet,  travels  at  the  same  rate  of  speed,  viz.,  the  almost 


102  ECLIPSES    OF    THE    SUN 

incredible  velocity  of  186,300  miles,  or  in  round  numbers 
300,000  kilometers  per  second.  To  simplify  the  calculation, 
suppose  the  light  is  violet,  of  wave-length  4000  A.  The 
length  of  these  waves  from  crest  to  crest  is  4000x10"'°, 
which  is  4.io~7  meters  (i.e.  4  divided  by  ten  million), 
300,000  kilometers  per  second  is  300,000,000  meters  01 
3.io8  meters  per  second.  If  therefore  we  divide  the  dis- 
tance that  light  travels  in  one  second  by  the  wave-length , 
we  will  find  the  number  of  waves.  For  light  of  4000  A, 
seven  hundred  and  fifty  millions  of  millions  (750,000,000,- 
000,000)  of  waves  enter  into  the  eye  in  one  second  of  time. 
If  the  light  under  consideration  is  red  instead  of  violet,  in- 
asmuch as  the  wave-length  of  the  red  is  longer  than  that 
of  the  violet,  fewer  red  waves  will  consequently  enter  into 
the  eye  in  a  given  time.  These  tiny  waves  impinge  on  the 
retina  of  the  eye,  creating  motions  which  when  telegraphed 
to  the  brain  cause  the  sensation  of  violet  or  red  light.  The 
mechanism  by  means  of  which  the  minute  motions  produced 
in  the  eye  by  light  waves  cause  the  sensation  of  light  has 
never  been  completely  discovered.  Professor  John  Joly  in 
Philosophical  Magazine,  42,  289,  1921,  gives  a  very  plaus- 
ible explanation  based  on  the  quantum  theory.  He  as- 
sumes that  the  origin  of  vision  and  color  perception  is  to 
be  sought  in  the  liberation  of  electrons  under  light  stimulus 
within  a  photoelectric  substance  or  substances  existing  in 
the  retina.  In  the  case  of  the  rods,  rhodopsin,  being  such 
a  photosensitive  substance,  acts  as  the  basis  of  vision,  and 
it  is  assumed  that  the  same  substance  in  the  cones  is  re- 
sponsible for  the  color  vision.  The  sensitivity  of  the  eye 
to  faint  light  is  extraordinary.  Henri  and  des  Baucels  have 
found  that  the  retina  is  sensitive  to  a  minute  amount  of 
light  'energy  which,  when  expressed  in  physical  units, 
amounts  to  5xio~12  erg.  The  quantum  for  green  light  is 
4xio~12  erg,  and  hence  it  is  assumed  that  one  quantum,  by 
the  liberation  of  a  single  electron,  is  sufficient  to  cause  the 
sensation  of  light.  The  action  taking  place  in  the  eye 
seems  to  be  quite  analogous  to  that  occurring  in  the  light- 
sensitive  film  of  the  photographic  plate,  the  latent  image 
being  caused  by  the  movement  of  electrons. 


THE    SPECTROSCOPE  103 

The  new  method  of  research  in  the  hands  of  Kirchhoff 
soon  resulted  (1861)  in  the  discovery  of  two  new  chemical 
elements,  caesium  and  rubidium.  The  spectroscope  manu- 
factured by  Steinheil  consisted  of  four  prisms,  three  of  45° 
and  one  of  60°.  The  collimator  and  telescope  had  aper- 
tures of  one  and  a  half  inches,  with  a  focal  length  of  eight 
een  inches.  One  half  of  the  slit  was  covered  by  a  totally 
reflecting  prism  by  the  aid  of  which  two  spectra  could  be 
examined  side  by  side  and  direct  comparisons  made.  Un- 
fortunately, it  was  necessary  to  set  to  minimum  deviation 
by  hand,  a  very  slow  process,  and  as  a  consequence  Kirch- 
hoff shifted  the  prisms  only  occasionally,  a  procedure  which 
greatly  impaired  his  measures.  He  investigated  the  spectra 
of  a  large  number  of  elements,  and  also  measured  the  posi- 
tions of  the  lines  in  the  solar  spectrum  from  A  to  G,  though 
at  first  he  only  published  the  region  from  D  to  F,  since  his 
eyes  could  not  stand  the  strain  of  such  continuous  measure- 
ment and  failed  him.  His  measures  being  referred  to  an 
arbitrary  scale,  it  was  necessary  to  reduce  them  to  wave- 
lengths, and  this  was  done  later  by  Airy,  Gibbs,  Watts  and 
Hasselberg. 

It  has  been  said  with  great  verity  that  these  were  splen- 
did days  for  the  laboratory  scientist  for  the  reason  that 
each  and  every  observation,  no  matter  how  trivial,  was 
almost  certain  to  prove  to  be  a  new  discovery.  Without 
attempting  to  trace  the  details  in  the  further  development 
of  the  new  science,  we  shall  try  to  give  only  the  more  im- 
portant names  in  the  honor  roll  of  fame:  Pliicker,  Hittorf, 
Crookes,  Miller,  Huggins,  Rutherfurd,  Angstrom,  Secchi, 
Janssen,  Lockyer,  Young,  Vogel,  Cornu,  Liveing  and  De- 
war.  In  the  twenty  years  following  Kirchhoff,  no  less  than 
ten  new  elements  were  found  by  the  aid  of  the  spectroscope. 
Naturally  many  mistakes  were  made  and  wrong  conclusions 
drawn.  The  chief  cause  of  the  mistakes  was  the  presence 
of  many  impurities  in  the  elements  investigated  and  hasty 
identification  of  lines  through  insufficient  accuracy  in  wave- 
length determinations. 

A  new  epoch  in  the  history  of  spectrum  analysis  was  in- 
augurated in  1882  by  the  work  of  Henry  A.  Rowland,  whose 


104  ECLIPSES    OF    THE    SUN 

gratings/  plane  and  concave,  permitted  a  hundred-fold  in- 
crease in  accuracy  in  the  determination  of  wave-lengths. 
Rowland's  success  came  through  the  construction  of  a  long 
screw,  almost  free  from  errors,  mounted  in  a  dividing  engine 
in  such  a  manner  that  it  was  practically  possible  to  elimi- 
nate the  few  remaining  errors  of  the  screw.  The  precision 
that  must  be  attained  in  the  manufacture  of  gratings  of 
the  very  first  quality  may  be  stated  as  one  requiring  that 
the  average  line  of  the  grating  shall  be  correctly  placed  to 
about  one  one-thousandth  part  of  the  grating  space.  This, 
for  the  finest  Rowland  gratings  of  20,000  lines  per  inch, 
means  that  each  line  ruled  on  the  grating  must  not  on  the 
average  differ  from  its  true  position  by  so  much  as  the 
minute  quantity  of  one  twenty-millionth  part  of  an  inch! 

The  highest  degree  of  success  was  attained  by  ruling  with 
a  diamond  point  on  speculum  metal,  the  incident  light  thus 
being  reflected  from  the  grating  surface.  Gratings  were 
ruled  both  on  plane  and  on  spherically  concave  surfaces. 
The  largest  Rowland  gratings  were  six  inches  in  diameter, 
and  ordinarily  the  greatest  radius  of  curvature  for  the  con- 
cave gratings  was  twenty-one  and  a  half  feet.  The  concave 
gratings  reduced  the  spectroscope  to  the  greatest  simplicity 
of  slit,  grating  and  photographic  plate.  No  lenses  of  any 
kind  were  necessary  to  bring  the  light  to  a  focus,  and  hence 
all  aberrations  introduced  by  the  lenses  and  all  absorption 
of  light  by  the  glass  were  eliminated,  with  a  consequent 
great  increase  in  the  extent  of  the  ultra-violet  region.  The 
concave  grating  is  generally  used  in  the  laboratory  with  the 
"  Rowland  mounting  "  possessing  two  tracks  for  carrying 
grating  and  photographic  plate  perpendicular  to  each  other, 
the  slit  being  placed  accurately  at  the  intersection  of  the  two 
tracks.  If  grating  and  photographic  plate  are  each  perpen- 
dicular to  the  arm  joining  the  two,  there  then  results  a  "  nor- 
mal "  spectrum,  or  one  in  which  the  distances  between  the 
lines  are  directly  proportional  to  the  wave-length. 

Compared  with  prisms,  concave  gratings  have  the  follow- 
ing advantages:  (i),  An  enormous  increase  in  dispersion, 
definition  and  resolving  power.  To  equal  Rowland's  grat- 

1  See  Kayser,  Handbuch  der  Spectroscopie,  Vol.  i,  121  and  397.  See  also 
article  Screw,  Encyclopaedia  Britannica. 


THE    SPECTROSCOPE  105 

ings  in  these  respects,  in  the  neighborhood  of  the  D  lines, 
it  would  be  necessary  to  have  prisms  of  the  very  first  quality 
added  to  prisms  with  a  total  prism  base  of  fifty  inches  of 
glass.  (2),  The  spectrum  produced  by  the  grating  is 
"  normal "  and  not  "  prismatic/'  thus  permitting  wave- 
lengths to  be  determined  with  much  greater  facility 
and  much  greater  accuracy.  (3),  A  much  greater  extent 
of  the  ultra-violet  is  secured.  (4),  The  astigmatism 
of  the  grating  increases  the  length  of  the  lines,  and  this 
principle,  combined  with  the  overlapping  of  images  from 
the  spectra  of  different  orders,  not  only  permits  a  great 
increase  in  accuracy,  but  also  a  more  ready  determina- 
tion of  absolute  wave-lengths.  The  grating,  however, 
has  some  disadvantages,  chief  among  which  is  that  the  in- 
cident light  is  divided  between  the  central  beam  and  many 
different  orders  of  spectra,  with  the  consequent  result  that 
there  is  a  great  weakening  of  light  in  any  one  spectrum. 
In  the  investigation  of  objects  giving  little  light,  like  the 
stars,  prisms  are  almost  universally  used.  In  most  labor- 
atory researches  and  in  work  on  the  sun,  gratings  are  gen- 
erally used.  Although  Rowland's  method  of  grinding  the 
screw  was  not  a  secret,  it  is  only  in  comparatively  recent 
times  that  the  excellence  of  the  Rowland  gratings  has  been 
equalled  by  others,  by  Michelson  of  the  University  of 
Chicago,  by  the  Mount  Wilson  Observatory,  and  by  Lyle 
and  Merfield  of  Australia. 

Coincident  with  Rowland's  manufacture  of  the  grating 
came  the  discovery  of  the  modern  photographic  dry  plate 
with  its  great  increase  in  sensitiveness.  The  two  most  im- 
portant pieces  of  work  in  this  new  epoch  of  discovery  in 
astrophysics  have  been  Rowland's  great  map  of  the  solar 
spectrum,  and  the  publication  in  the  Astrophysical  Journal 
of  the  wave-lengths  of  the  lines  in  the  solar  spectrum  with 
the  tracing  of  as  many  lines  as  possible  to  their  chemical 
origins.  The  more  prominent  names  connected  with  inves- 
tigations in  the  laboratory  and  on  the  sun  itself  are:  Row- 
land, Jewell,  Kayser,  Runge,  Paschen,  Rydberg,  Eder  and 
Valenta,  Exner  and  Haschek,  Langley,  Abbot,  Schumann, 
Lyman,  Humphreys,  Zeeman,  Hale,  Deslandres,  St.  John, 
King,  Meggers,  Russell,  Saunders  and  Fowler. 


CHAPTER  VII 

THE    SURFACE    OF   THE    SUN 

AT  THE  time  of  the  Greeks  how  simple  it  was  to 
explain  all  of  the  known  facts  about  the  sun!     No 
supposition  was  necessary  other  than  that  the  sun 
was  a  ball  of  fire.     It  was  not  at  all  known  what  fire  was 
beyond  the  fact  that  heat  was  manifested,  —   but  this  de- 
ficiency in  knowledge  seemed  of  little  importance.     As  a 
further  illustration  of  the  elemental  beliefs  of  primitive 
peoples  there  might  be  given  the  following  legend  regard- 
ing the  origin  and  motion  of  the  sun  which  is  found  among 
the  Yuki  tribe  of  American  Indians.1 

"  In  the  beginning  there  was  no  land;  all  was  water. 
Darkness  prevailed  everywhere.  Over  this  chaos  of  dark 
water  hovered  On-coye-to  who  appeared  in  the  form 
of  a  beautiful  white  feather,  hence  the  love  of  the  Yukis 
for  feathers.  In  time  the  spirit  became  weary  of  his  inces- 
sant flight  through  the  murky  space  and  lighted  down  upon 
the  face  of  the  water.  Where  he  came  in  contact  there  was 
a  whirlpool  that  spun  his  body  round  and  round.  So  rapid 
became  the  motion  that  a  heavy  foam  gathered  about  him. 
This  became  more  dense  and  expanded  in  width  and  length. 
It  gathered  up  the  passing  bubbles  until  it  was  a  huge  float- 
ing island.  On  the  bosom  of  this  rested  the  snowy  form 
of  On-coye-to.  As  he  lay  upon  this  island  for  an  almost 
endless  flight  through  the  dark  space,  the  idea  of  a  perma- 
nent resting  place  came  into  his  mind.  So  he  made  the  land 
and  divided  it  from  the  water.  From  the  form  of  a  feather 
he  assumed  that  of  a  man,  and  rested  upon  the  land.  Still 
there  was  no  light,  and  his  spirit  was  troubled.  On-coye- 
to  saw  afar  off  in  the  firmament  a  star,  '  po-ko-lil-ey/  and 
resolved  to  visit  it  and  learn  how  it  emitted  its  sparkling 

1  Smithsonian   National    Mus.    Report,   326,    1902. 
106 


THE    SURFACE    OF    THE    SUN  107 

light.  After  a  long  journey,  he  arrived  there  and  found  a 
large  and  beautifully  lighted  world,  inhabited  by  a  numer- 
ous, hospitable  people.  Still,  he  saw  not  whence  came  the 
light.  He  was  allowed  free  access  to  all  habitations  save 
one,  the  '  sweat  house.7  This  was  guarded  night  and  day, 
and  was  accessible  only  to  sick  persons.  Finally  a  great 
hunt  was  planned,  and  as  time  drew  near  all  was  prepared 
for  the  occasion.  But  On-coye-to  feigned  sickness  that 
he  might  investigate  the  sweat  house.  When  the  morning 
arrived  for  the  hunt  he  was  too  ill  to  acompany  the  hunters. 
A  council  was  held  to  determine  whether  this  stranger 
should  be  admitted  to  the  sweat  house,  which  is  even  now 
a  sacred  place  with  the  Yuki  tribe,  and  it  was  decided  to 
give  him  the  benefit  of  this  house  of  medicine.  A  few  old 
men  were  left  to  administer  to  his  wants  and  to  see  that 
all  went  well.  As  he  entered  the  sweat  house  he  was  almost 
blinded  by  the  light  that  flashed  upon  him,  but  as  he  became 
accustomed  to  it,  he  looked  around  him  and  discovered  its 
origin.  Hanging  high  over  his  head  in  several  baskets  were 
as  many  beautiful  suns.  Having  found  the  fountain  of 
light  he  waited  patiently  until  the  old  men  were  all  asleep, 
then  climbing  cautiously  to  what  seemed  the  brightest  of 
the  suns,  he  seized  it,  slipped  from  the  sweat  house  and 
made  his  way  rapidly  towards  his  own  world.  He  was 
hotly  pursued  by  the  indignant  warriors,  but  he  arrived 
safely  after  many  adventures.  He  hung  the  sun  in  its 
basket  in  the  far  east,  then  surveyed  it.  It  did  not  light  up 
to  suit  him,  and  he  moved  it  a  little  higher.  Still  it  did 
not  suit  him,  so  he  continued  to  move  it,  on  and  on.  And 
he  is  moving  it  to  the  present  day."  Thus  the  Indian  ac- 
counts for  the  moving  of  the  sun,  and  thinks  not  that  the 
earth  moves. 

As  knowledge  has  gradually  been  accumulated  regard- 
ing our  central  luminary,  and  as  information  is  secured 
about  the  laws,  physical  and  chemical,  to  which  the  sun  is 
subjected,  the  more  and  more  difficult  has  become  the  prob- 
lem of  finding  an  explanation  adequate  to  satisfy  all  of  the 
facts.  And  now  in  the  twentieth  century,  it  is  discovered  that 
each  and  every  one  of  the  countless  billions  of  chemical  atoms 


108  ECLIPSES    OF    THE    SUN 

that  form  the  sun  is  a  solar  system  in  miniature,  with  the 
result  that  solar  theories  must  be  revised  and  hypotheses 
revamped  in  order  to  take  account  of  electrons  and  protons. 
The  sun  is  a  typical  star,  but  owing  to  its  proximity  it  may 
be  examined  in  detail,  its  surface,  the  spots  that  are  of 
such  great  interest,  the  reversing  layer,  the  chromosphere 
and  the  far-flung  corona.  The  stars  are  so  far  distant  that 
they  appear  practically  as  points  of  light  even  in  our  larg- 
est and  best  telescopes,  and  consequently  little  more  can 
be  learned  of  them  than  what  depends  on  their  surface 
brightness.  On  account  of  the  closeness  of  the  sun,  its  sur- 
face may  be  covered  up  by  the  interposing  moon,  and  as 
a  result,  envelopes  of  chromosphere  and  corona  are  shown. 
Although  similar  envelopes  unquestionably  exist  on  the  stars, 
they  can  never  be  made  manifest  to  us.  By  the  study  of 
eclipses  a  wealth  of  knowledge  is  acquired  concerning  the 
sun,  but  to  gain  an  adequate  idea  of  what  additional  in- 
formation is  thus  secured,  it  will  be  well  to  give  a  brief 
resume  of  the  salient  points  of  solar  research. 

Compared  with  terrestrial  standards,  the  distance  to  the 
sun  is  colossal  and  its  diameter  enormous.  The  problem  of 
finding  the  distance  of  the  sun  is  one  of  the  most  important 
as  well  as  one  of  the  most  difficult  in  the  whole  of  the 
science  of  astronomy.  The  importance  lies  in  the  fact  that 
the  distance  from  earth  to  sun  is  the  unit  for  measuring  all 
celestial  distances,  except  that  to  the  moon,  the  solar  dis- 
tance being  called  the  astronomical  unit.  The  yard,  or  the 
meter,  is  the  standard  of  length  for  all  measurements  in 
civilized  countries.  The  "  standard  yard/'  or  the  "  standard 
meter,"  is  a  bar  of  certain  composition,  of  definite 
shape,  whose  length  can  be  determined  by  precise  measure- 
ments at  known  temperatures.  The  standards  are  kept  in 
London  or  Paris,  but  various  prototypes  are  widely  dis- 
tributed. Certain  advantages  would  result  from  the  em- 
ployment of  but  one  standard  of  length,  the  meter,  which 
is  now  almost  exclusively  used  in  all  scientific  measure- 
ments. The  reason  is  not  because  the  length  of  the  meter 
is  more  valuable  than  that  of  the  yard,  but  rather  that  the 
decimal  system  employed  with  the  meter  is  simpler  than  the 


THE  SUN  IN  THE  LIGHT  OF  GLOWING  CALCIUM  VAPOR 
Two  spectroheliograms  showing  unusual  solar  activity.    Yerkes  Observatory. 


X 

I 


fO 

& 
X 

Ctf 

3 


THE    SURFACE    OF    THE    SUN  109 

more  cumbrous  division  of  the  yard.  At  the  end  of  the 
eighteenth  century  the  meter  was  designed  by  the  French 
to  represent  the  ten-millionth  part  of  the  quadrant  of  the 
earth,  so  that  its  circumference  should  be  exactly  forty 
million  meters.  At  that  time,  however,  the  size  of  the  earth 
was  known  with  little  accuracy.  If,  therefore,  the  meter 
were  actually  to  represent  a  certain  definite  and  fixed 
fraction  of  the  earth's  size,  it  would  be  impossible  to  use 
it  as  a  standard  for  the  reason  that  its  length  would  change 
with  every  revision  of  the  earth's  measurement  as  new 
geodetic  operations  were  carried  out,  and  would  alter  with 
any  variations  in  the  earth  itself,  which  changes  geodesy 
and  geology  tell  us  are  continually  taking  place. 

By  means  of  the  careful  researches  of  mathematical  as- 
tronomy stretching  back  over  hundreds,  and  even  thousands 
of  years,  an  accurate  plot  can  be  drawn  to  scale  of  the 
orbits  of  all  the  planets  and  satellites  of  the  solar  system. 
The  periods  of  each  have  been  determined  accurately,  the 
shapes  of  their  orbits,  their  inclinations  to  the  ecliptic,  etc. 
All  of  the  planetary  distances  have  been  found  by  the  as- 
tronomer by  referring  them  to  the  astronomical  unit,  the 
distance  from  earth  to  sun.  To  know  the  complete  scale 
of  the  astronomical  plan  in  miles,  it  is  necessary  to  know 
accurately  at  least  one  distance,  either  that  of  the  earth  to 
the  sun,  or  to  one  of  the  planets.  Manifestly,  the  nearer 
the  planet,  the  more  accurately  can  the  distance  from  the 
earth  be  determined.  Celestial  distances  are  usually  found 
by  the  methods  of  the  surveyor  who  wishes  to  ascertain  the 
width  of  a  river  which  he  cannot  traverse.  A  base-line  is 
measured  with  as  great  precision  as  possible,  and  from  each 
end  of  this  base-line,  angles  are  measured  to  some  well- 
defined  object  on  the  other  side  of  the  river.  A  civil 
engineer  would  be  in  a  quandary  if  a  base-line  of  only  three 
inches  in  length  was  available  for  measurement  on  his  side 
of  the  river  and  it  was  necessary  to  determine,  with  the  pre- 
cision necessary  for  planning  a  cantilever  bridge,  the  dis- 
tance across  the  river.  This  is  exactly  the  problem  that  con- 
fronts the  astronomer  in  attempting  to  measure  directly  the 
distance  of  the  sun,  since  the  only  base-line  available  is  a 


110  ECLIPSES    OF    THE    SUN 

chord  of  the  earth.  Instead  of  expressing  the  distance  to 
the  sun  in  miles  or  kilometers,  the  astronomer  knows  this 
unit  ordinarily  by  means  of  the  small  angle  at  the  sun  sub- 
tended by  the  earth's  radius,  and  this  angle  is  called  the 
"  solar  parallax."  As  there  are  many  different  terrestrial 
radii,  the  earth  not  being  a  sphere,  the  equatorial  radius  is 
assumed.  The  astronomical  unit  when  expressed  in  miles 
must  therefore  be  subject,  not  only  to  all  of  the  errors  of  the 
astronomer  in  carrying  out  his  measurements,  but  also  to 
those  due  to  the  work  of  the  geodesist  in  determining  the 
shape  and  size  of  the  earth  and  in  referring  these  measures 
to  the  standard  yard  or  meter. 

Although  this  is  not  the  place  to  discuss  the  details  of 
the  determination  of  the  solar  parallax,  brief  references 
will  be  made  to  the  more  promising  methods.  It  is  im- 
possible to  secure  the  distance  to  the  sun  by  direct  measures 
and  recourse  must  be  had  to  the  determination  of  this  dis- 
tance indirectly  by  the  measurement  of  other  distances  in 
the  solar  system.  Great  were  the  expectations  aroused  by 
the  transits  of  Venus  in  1761  and  1769,  and  again  in  1874 
and  1882.  By  the  year  1882,  the  dry  plate  had  been 
invented  and  it  was  anticipated  that  photography  would 
revolutionize  our  knowledge  of  the  fundamental  unit.  But 
alas!  the  "  black  drop,"  and  the  atmosphere  of  Venus 
caused  the  observations  to  be  practically  a  dismal  failure. 
Another  attempt  by  this  method  will  not  be  possible  until 
the  year  2004,  the  year  of  the  next  transit  of  Venus. 

Great  advances  in  precision  were  made  by  Gill  in  his 
measures  of  Mars  and  some  of  the  minor  planets  by  means 
of  the  heliometer.  The  discovery  in  1898  of  the  planetoid 
Eros,  which  at  perihelion  comes  closer  to  the  earth  even 
than  Mars,  gave  to  the  astronomer  a  splendid  opportunity 
of  determining  the  solar  parallax  for  the  reason  that 
photography  could  be  applied  to  the  problem.  Although 
much  was  accomplished  at  the  opposition  of  1900-01,  when 
Eros  at  its  nearest  approach  was  30,000,000  miles  from 
the  earth,  more  will  be  effected  from  observations  made  in 
1931  when  the  planet  was  within  half  this  distance. 

Methods  based  on  the  law  of  gravitation  furnish  the 


THE    SURFACE    OF    THE    SUN  111 

means  of  determining  the  distance  of  the  sun.  E.  W. 
Brown's  magnificent  investigation  of  the  motion  of  the 
moon  gives  8."778  as  the  value  of  the  solar  parallax.  Other 
gravitational  methods  furnish  the  mean  value  of  8. "780. 

The  velocity  of  light  can  be  utilized  in  three  different 
manners:  (a)  by  the  constant  of  aberration;  (b)  by  the 
eclipse  of  Jupiter's  satellites;  and  (c)  by  the  velocity  of 
the  earth  in  its  orbit  by  utilizing  the  Doppler  principle  of 
measuring  the  motion  in  the  line  of  sight  from  spectra  of 
stars  or  of  planets.  St.  John  and  Nicholson  at  Mount 
Wilson,  using  large  dispersion  on  the  spectrum  of  Venus, 
find  the  solar  parallax  of  8."8i3.  (Publications  A.  S.  P., 
32,  332,  1920.)  They  give  also  the  values  by  other  ob- 
servers. 

A  summary  of  the  best  values  *  of  the  solar  parallax  (see 
Abbot,  The  Sun}  are: 

From  heliometer  work  on  minor  planets  8."8o7 

From  the  Eros  campaign  8.  807 

From  all  gravitational  methods  8.  780 

From  the  eclipses  of  Jupiter's  satellites  8.  799 

From  the  velocity  of  light  and  constant  of  aberration  (2o"47)     8.  803 

The  present  accepted  value  of  the  solar  parallax  is  8. "80, 
corresponding  to  a  distance  of  the  sun  of  92,900,000  miles, 
with  an  uncertainty  of  about  50,000  miles,  or  an  error  of 
about  one  part  in  2000.  To  walk  the  distance  to  the  sun  at 
four  miles  per  hour  and  ten  hours  per  day,  68  years  would 
be  necessary  for  the  first  million  miles,  or  6300  years  for  the 
total  distance.  An  express  train  going  at  sixty  miles  per 
hour  would  take  175  years,  while  light  which  travels  at  the 
great  speed  of  186,300  miles  per  second  takes  499  seconds. 

If  we  know  the  angular  diameter  of  the  sun  we  can  readily 
find  its  linear  diameter  of  865,000  miles,  which  is  109.5 
times  the  diameter  of  the  earth.  Perhaps  the  best  method 
of  visualizing  the  huge  size  of  the  sun  is  to  compare  it  with 
the  distance  to  the  moon,  which  in  round  numbers  is  239,000 
miles.  If  the  earth  could  be  placed  at  the  center  of  the  sun 
and  the  moon  were  allowed  to  revolve  in  her  orbit  about  the 

1  See  also,  Newcomb-Engelmann,  Populdre  Astronomic. 


112  ECLIPSES    OF    THE    SUN 

earth  there  would  be  plenty  of  room  inside  the  sun  for  the 
moon  to  make  her  monthly  journey,  since  the  moon  would 
be  little  more  than  half  way  out  to  the  sun's  surface.  A 
spot  on  the  sun  having  a  diameter  of  8000  miles,  or  the  size 
of  the  earth,  would  be  regarded  as  fairly  small  and  a  tele- 
scope would  be  needed  to  detect  it.  Dividing  the  linear 
diameter  of  865,000  miles  by  the  angular  diameter  1920", 
it  is  found  that  i"  at  the  sun  corresponds  to  450  miles,  which 
is  equivalent  to  725  kilometers.  These  are  useful  quantities 
to  remember,  especially  when  considering  the  subject  of 
eclipses. 

Since  the  surfaces  of  spheres  are  proportional  to  the 
squares,  and  the  volumes  proportional  to  the  cubes  of  their 
radii,  it  is  readily  found,  by  squaring  and  cubing  109.5,  that 
the  surface  of  the  sun  is  12,000  times  that  of  the  earth  while 
its  volume  is  1,300,000  times  the  volume  of  the  earth. 

The  mass  of  the  sun  is  332,000  times  the  mass  of  the 
earth,  a  relation  that  can  be  determined  by  comparing  the 
distance  a  body  falls  towards  the  earth  in  a  second  of  time 
in  obedience  to  the  law  of  gravitation  with  the  distance  that 
the  earth  falls  towards  the  sun  in  the  same  interval  of  time 
and  also  in  obedience  to  the  law  of  gravitation.  In  one 
second  the  earth  travels  eighteen  and  a  half  miles  of  her 
annual  journey  about  the  sun,  but  as  this  is  accomplished 
without  friction  we  feel  no  sensation  from  this  rapid  flight. 
In  going  eighteen  and  a  half  miles,  the  earth  deviates  but 
one-ninth  of  an  inch  from  a  straight  line.  As  the  earth 
weighs  six  thousands  of  millions  of  millions  of  millions  of 
tons,  which  is  6  x  io21  tons,  the  sun  weighs  2  x  id27  tons. 
This  is  such  a  colossal  number  that  it  makes  little  differ- 
ence in  our  comprehension  of  it  whether  it  is  the  American 
ton  of  2000  pounds,  or  the  English  ton  of  2240  pounds,  or 
the  long  ton  that  we  pay  for  when  we  buy  coal  or  the  short 
ton  that  is  furnished  by  the  dealer  when  the  coal  is  placed 
in  the  bin. 

Knowing  the  mass  and  volume  of  the  sun  compared  with 
that  of  the  earth,  we  find  the  density  of  the  sun  is  0.255  times 
that  of  the  earth,  or  about  1.4  times  the  density  of  water. 
The  attraction  of  gravity  at  the  surface  of  the  sun  is  27.6 


THE    SURFACE    OF    THE    SUN  113 

times  that  which  it  is  at  the  earth's  surface,  so  that  a  man 
weighing  150  pounds  would  weigh  over  two  tons  if  trans- 
ported to  the  sun,  and  his  feet  would  be  so  heavy,  even  if 
the  footing  were  secure,  that  he  would  not  have  strength 
sufficient  to  lift  them. 

The  small  density  of  the  sun,  being  only  one-quarter  that 
of  the  earth,  is  one  of  the  most  significant  bits  of  knowledge 
connected  with  the  study  of  the  sun.  All  theories  of  evolu- 
tion point  to  the  fact  that  the  sun  and  earth  are  made  of  the 
same  materials.  Indeed  Rowland  was  wont  to  say  that  if 
the  earth  were  heated  to  incandescence  it  would  give  a  spec- 
trum identical  with  that  of  the  sun.  The  low  density  makes 
it  evident  that  the  sun  cannot  be  a  solid  like  the  earth,  nor 
indeed  can  it  be  a  liquid,  and  it  must  therefore  be  a  gas,  the 
terrific  heat  of  the  sun  being  sufficient  to  vaporize  all  known 
terrestrial  substances.  The  condition  of  immense  heat  and 
enormous  pressure  caused  by  gravitation  on  the  sun  cannot 
be  even  distantly  approximated  in  our  laboratory  experi- 
ments. Unquestionably  the  sun  does  not  obey  the  labora- 
tory laws  to  which  such  perfect  gases  as  oxygen,  nitrogen 
and  hydrogen  are  subjected.  For  the  complete  explanation 
of  solar  phenomena  it  is  necessary  to  proceed  from  known 
conditions  to  those  impossible  to  duplicate  in  the  laboratory 
by  the  difficult  and  uncertain  methods  of  extrapolation. 
The  steps  of  scientific  development  must  accordingly  be 
carefully  planned  and  wisely  thought  out,  or  else  the  path 
of  truth  may  lead  away  from  the  goal  of  progress  rather 
than  towards  it. 

The  spherical  portion  of  the  sun  that  we  see  is  called  the 
photosphere.  According  to  Young,  The  Sun,  page  109,  the 
"  photosphere  is  a  sheet  of  self-luminous  cloud;  possibly  like 
the  clouds  of  our  own  atmosphere,  with  the  exception  that 
the  droplets  of  water  which  constitute  terrestrial  clouds  are 
replaced  in  the  sun  by  drops  of  molten  metal,  and  that  the 
solar  atmosphere  in  which  they  float  is  the  flame  of  a  burn- 
ing fiery  furnace,  raging  with  a  fury  and  an  intensity  beyond 
all  human  conception."  This  notion  of  the  photosphere 
propounded  a  third  of  a  century  ago  has  been  greatly 
modified  by  modern  research.  By  means  of  convection  cur- 


114  ECLIPSES    OF    THE    SUN 

rents,  gases  from  the  interior  of  the  sun  are  brought  to  the 
surface.  There  set  free  from  the  enormous  internal  pressure 
and  meeting  the  cooler  temperatures  of  outside  space  the 
gases  expand.  The  pent-up  energy  being  suddenly  released, 
there  is  a  rapid  fall  of  temperature,  and  according  to  Young's 
theory,  small  solid  or  liquid  particles  are  formed.  By  gravity 
these  sink  back  to  the  solar  furnace,  there  to  be  changed 
again  to  the  gaseous  form.  The  rising  and  falling  back 
again  to  the  solar  surface  of  the  "  drops  of  molten  metal  " 
cause  a  continued  rain  of  meteors  on  the  sun's  surface. 
Two  decades  ago  these  "  drops  "  played  a  very  important 
role  in  many  solar  theories,  but  particularly  in  that  regard- 
ing radiation  pressure  (Chapter  XXI).  The  temperature  of 
the  photosphere  "  seems  to  be  certainly  in  excess  of  6000° 
absolute,  Centigrade.  There  are  no  substances,  so  far  as 
known,  which  can  exist  except  as  vapors  in  these  conditions. 
Hence,  it  seems  reasonable  to  suppose  that  the  sun  contains 
no  solids  or  liquids,  unless  perhaps  in  sun-spots,  and  that  its 
substance,  as  we  see  it,  and  within  the  layers  we  see,  is 
altogether  gaseous."  l 

In  carrying  out  investigations  regarding  the  sun's  surface 
there  are  two  points  of  view  that  astronomers  should  never 
forget.  The  first  is  that  the  photosphere  can  be  viewed  or 
photographed  only  through  the  superposed  layers  of  the 
solar  atmosphere.  The  photospheric  spectrum  which  must 
be  continuous  from  red  to  violet  without  breaks,  can  never 
be  obtained.  The  second  point  to  be  remembered  may  be 
visualized  by  analogy  with  the  earth.  At  an  elevation  of 
three  and  a  half  miles  above  sea-level,  atmospheric  air  has 
its  density  cut  in  half.  But  gravity  at  the  sun  is  nearly 
twenty-eight  times  its  value  on  the  surface  of  the  earth. 
Allowance  being  made  for  the  hundred-fold  diameter  of  the 
sun  when  compared  with  that  of  the  earth,  it  is  seen  that 
within  ten  or  twelve  miles  of  the  sun's  surface  there  would 
exist  one-half  of  the  total  material  in  the  sun's  various  layers 
of  gases  were  it  not  for  the  enormous  temperature  of  the 
sun.  The  decrease  in  pressure  upwards  from  the  sun's 
surface  is  consequently  extremely  rapid.  The  importance  of 

1  Abbot,  The  Sun,  p.  243,  1911. 


THE    SURFACE    OF    THE    SUN  115 

this  point  cannot  be  over-emphasized,  since  most  investi- 
gators seem  to  forget  that  the  change  in  pressure  must  take 
place  at  such  a  very  accelerated  rate. 

The  sun  may  be  viewed  with  a  telescope  by  the  use  of 
solar  eye-pieces  of  various  kinds;  or  by  projecting  the  sun 
on  a  screen,  the  solar  image  being  brought  to  a  focus  by 
slightly  drawing  out  the  telescopic  ocular.  If  the  telescope 
is  of  moderate  size,  and  the  definition  good,  the  surface  of 
the  sun  looks  like  "  rough  drawing  paper,  or  like  curdled 
milk  seen  from  a  little  distance."  The  use  of  a  large  tele- 
scope and  of  moments  of  exquisite  seeing  that  come  but 
rarely  reveal  an  infinite  wealth  of  detail.  The  best  drawings 
of  the  sun  are  by  Langley  who  describes  the  surface  of  the 
sun  as  that  of  "  snow-flakes  sprinkled  sparsely  over  a  grayish 
cloth."  Before  the  application  of  photography  great  was 
the  diversity  of  opinion  concerning  the  ultimate  nature  of 
the  light-giving  particles  of  the  sun.  We  learned  then  of 
"  rice  grains,"  of  "  willow  leaves,"  of  "  thatch-straw  "  and 
of  "  granules  ";  and  the  various  camps  in  favor  of  one  or 
other  designation  were  about  equally  divided. 

Photographs  of  the  sun  may  be  obtained  by  the  method 
known  to  the  great  host  of  camera  users,  that  of  the  focal- 
plane  shutter.  This  consists  essentially  of  a  slit  of  variable 
size  that  can  be  driven  by  means  of  a  spring  across  in  front 
of  and  close  to  the  photographic  plate.  By  regulating  the 
width  of  the  slit  opening  and  the  tension  of  the  spring, 
exposures  may  be  varied  at  will.  The  exposures  necessary 
to  obtain  good  photographs  of  the  sun  depend  mainly 
on  the  size  of  the  telescope,  the  sensitivity  of  the 
photographic  plate  and  the  method  of  development.  The 
modern  dry  plate  of  great  rapidity  does  not  permit  the  se- 
curing of  solar  photographs  of  the  greatest  detail.  With  the 
sun,  where  there  is  such  an  abundance  of  light,  it  is  unneces^ 
sary  to  make  use  of  the  fastest  plates  which  are  primarily  fot 
the  purpose  of  decreasing  the  exposures.  Better  results  may 
be  secured  by  the  use  of  finer  grained,  slower  and  more  con- 
trasty  plates.  As  a  matter  of  fact,  the  modern  dry  plate 
cannot  furnish  the  exquisite  definition  secured  by  the  old 
wet-plate  process,  and  for  this  reason  the  superb  solar  photo- 


116  ECLIPSES    OF    THE    SUN 

graphs  of  Janssen  at  Meudon  are  unsurpassed  even  at  the 
present  day. 

The  best  conditions  for  observing  the  sun  are  found  not 
more  than  one  per  cent  of  the  time  spent  at  the  telescope. 
Under  these  maximum  conditions,  the  skilled  eye  can  see 
finer  details  than  can  be  portrayed  on  the  photographic 
plate.  According  to  Langley,  the  "  snow-flakes  "  are  in  the 
neighborhood  of  50  to  100  miles  in  diameter,  and  these  in 
turn  are  made  of  flakes  similar  in  form,  but  of  one-fifth  the 
dimensions.  These  small  particles  cover  but  one-fifth  of 
the  surface  but  radiate  three-quarters  of  the  total  solar  light, 
and  hence  they  must  shine  with  an  intensity  twenty-fold 
that  of  the  darker  portions  of  the  sun. 

The  most  noticeable  feature  of  photographs  showing  the 
whole  solar  disk  is  the  darkening  that  is  found  near  the  edge 
of  the  sun.  This  darkening  is  caused  by  the  absorption  by 
the  sun's  atmosphere,  a  beam  from  the  limb  of  the  sun  pass- 
ing through  a  greater  layer  than  one  from  the  center.  The 
sun  or  moon  when  rising  or  setting  looks  reddish  to  us  on 
account  of  the  absorption  of  the  blue  and  violet  by  our 
terrestrial  atmosphere.  In  a  similar  manner,  the  solar  atmos- 
phere absorbs  more  and  more  of  the  violet  end  of  the  spec- 
trum as  the  limb  of  the  sun  is  approached,  and  the  maximum 
of  radiation  is  displaced  towards  the  red.  A  similar  shifting 
of  the  wave-length  maximum  is  found  when  comparing  the 
spectrum  of  a  sun-spot  with  that  of  the  photosphere.  Abbot 
(The  Sun,  page  107)  gives  measures  of  the  distribution  of 
radiation  over  the  sun's  disk  from  the  center  outwards  to 
the  edge.  If  the  sun  could  be  viewed  without  the  absorptive 
effects  of  its  own  and  the  earth's  atmospheres,  the  maximum 
intensity  in  the  spectrum  would  be  shifted  by  an  appreciable 
amount  to  the  violet,  and  the  sun  instead  of  appearing  as 
yellow  in  color  to  the  eye  would  look  bluish. 

The  exquisite  photographs  of  Janssen  show  the  solar 
granulation  in  splendid  detail,  the  features  being  sharp  and 
well-defined.  Other  parts  of  the  same  photographs  are  quite 
smudgy  in  comparison,  as  if  the  solar  surface  were  in  violent 
commotion.  To  these  parts  Janssen  gave  the  name  reseau 
photospherique.  If  photographs  taken  in  rapid  succession 


THE  SUN'S  SURFACE 

"  Like  snowflakes  sprinkled  over  a  grayish  cloth." 
Photographed  by  Janssen  at  Meudon,  September  9,  1883. 


THE    SURFACE    OF    THE    SUN  117 

are  examined  it  is  found  that  the  smudgy  and  ill-defined  por- 
tions exist  at  different  parts  of  the  solar  image.  The 
simplest  and  most  apparent  explanation  seemed  to  be  that 
these  changes  afforded  positive  evidence  of  violent  commo- 
tion on  the  sun  which  certainly  must  exist  there  on  account 
of  the  very  high  temperature.  This  evident  explanation, 
however,  seems  not  to  be  the  true  one.  Any  motion  of  the 
atmosphere  of  the  sun,  or  of  the  earth's  atmosphere  close  to 
or  far  away  from  the  photographic  plate  would  have  the 
effect  of  blurring  the  photographic  image.  To  make  a  long 
story  short  —  the  general  opinion  regarding  the  reseau 
photospherique  is  that  it  is  not  a  solar  phenomenon  at  all, 
but  is  caused  by  the  disturbance  of  the  air  heated  in  the 
telescopic  tube  by  the  sun's  beams.  The  portions  of  the 
photograph  in  good  definition  represent  the  true  granulation 
of  the  solar  surface.  Direct  photographs  similar  to  those  of 
Janssen  have  been  made  by  Hansky  of  Poulkova  and 
Chevalier  of  the  Zo-Se  Observatory  in  China.  Exposures 
made  in  rapid  succession  give  the  following  information  re- 
garding the  ultimate  nature  of  the  photosphere  as  depicted 
by  photographs:  (i),  The  solar  granules  have  a  diameter 
of  400  to  1 200  miles,  though  at  times  smaller  granules  are 
seen  no  more  than  100  miles  in  diameter.  (2),  They  are 
generally  circular,  or  elliptical  in  shape.  (3) ,  They  coalesce 
to  form  larger  particles.  (4),  These  granules  are  the 
"  clouds  "  of  Young's  theory.  (5),  The  life  of  one  of  these 
clouds  is  very  short,  the  majority  of  them  last  for  approxi- 
mately half  a  minute,  and  practically  none  exist  longer 
than  a  few  minutes.  (6),  The  displacements  vary  widely 
in  direction  and  in  velocity.  The  movements  range  from 
zero  to  thirty  kilometers  per  second,  though  occasionally 
higher  speeds  are  observed.  (7),  The  granules  in  fact  seem 
to  be  the  summits  of  a  fleecy  structure  of  condensed  par- 
ticles. In  fact,  they  represent l  on  an  enormous  scale  a 
phenomenon  similar  in  appearance  to  a  storm-tossed  and 
choppy  sea  when  viewed  aloft  from  an  airplane. 

The  most  prominent  features  of  the  solar  surface  are  the 
spots.    Individual  records  of  these  exist  as  far  back  as  the 

1  Astrophysical  Journal,  27,  12,  1908. 


118  ECLIPSES    OF    THE    SUN 

Chinese,  but  their  real  history  begins  with  the  invention  of 
the  telescope;  and  they  were  independently  discovered  by 
Galileo,  Fabricius  and  Scheiner.  There  is  a  great  wealth  of 
scientific  literature  connected  with  the  study  of  spots  —  but 
here  it  will  be  possible  to  give  only  the  salient  features,  and 
a  brief  summary  of  our  present  knowledge,  which,  alas!  is 
far  from  complete.  As  in  the  study  of  the  photosphere,  the 
details  of  the  appearance  of  spots  can  be  better  observed 
visually  than  by  photography.  A  normal  spot  consists  of 
an  umbra,  more  or  less  round,  surrounded  by  a  less  darkened 
penumbra,  the  structure  of  the  constituent  parts  of  a  spot 
differing  much  from  each  other  and  from  the  surface  of  the 
photosphere.  The  roundish  shapes  of  the  granular  photo- 
sphere are  changed  in  appearance  to  the  straw-thatch  of  the 
penumbral  filaments  which  exhibit  a  great  wealth  of  detail, 
and  these  in  turn  transform  into  the  smooth,  black,  velvet- 
like  appearance  of  the  umbra.  The  umbra  is,  however,  not 
uniformly  black  but  is  more  or  less  cloudy  in  appearance 
when  conditions  of  seeing  are  at  the  best.  Generally  asso- 
ciated with  spots  are  the  jaculae,  or  bright  patches  on  the 
sun.  These  exist  at  slight  elevations  above  the  average  sur- 
face of  the  sun,  and  are  best  seen  when  near  the  edge  of  the 
sun  where  the  greater  absorption  of  the  sun's  atmosphere 
and  the  elevation  of  the  faculae  make  them  visible  by  con- 
trast with  the  darker  surroundings.  The  umbra  of  a  spot 
is  dark  only  by  contrast  with  the  more  dazzling  photosphere, 
yet  withal  it  is  not  black,  for  it  is  more  brilliant  than  the 
electric  arc.  During  the  progress  of  an  eclipse  of  the  sun 
a  spot  has  been  observed  by  Evershed  to  be  much  brighter 
than  the  dark  limb  of  the  moon  occulting  it.  Langley  esti- 
mates that  the  blackest  spot  gives  500  times  as  much  light 
as  an  equal  area  of  the  full  moon. 

Spots  vary  in  size,  from  a  few  hundred  miles  to  50,000 
miles  in  diameter  in  the  case  of  the  very  largest  spots. 
Groups  of  spots  may  extend  across  one-sixth  of  the  diameter 
of  the  sun,  and  consequently  may  be  visible  to  the  naked  eye 
when  the  sun  is  seen  through  haze  or  near  the  horizon,  or 
when  the  eyes  are  protected  by  smoked  glass.  Spots  have 
usually  a  short  life,  sometimes  disappearing  in  a  day  or  two, 


THE    SURFACE    OF    THE    SUN  119 

sometimes  lasting  for  a  month  or  longer.  The  longest  record 
is  that  of  the  spot  seen  during  the  years  1840-41  which  per- 
sisted for  eighteen  months.  Owing  to  the  violent  solar  mo- 
tion the  changes  in  sun-spots  are  naturally  very  rapid,  the 
disintegration  of  spots  taking  place  usually  by  the  formation 
of  a  bright  "  bridge  "  which  may  be  shot  across  a  spot  at  a 
high  rate  of  speed  (compared  with  terrestrial  motions) ,  of 
as  much  as  one  thousand  miles  per  hour.  The  elevation  of 
sun-spots  with  respect  to  the  general  photospheric  level  is 
still  being  actively  discussed,  even  after  the  lapse  of  a  cen- 
tury and  a  half  since  1769,  when  Dr.  A.  Wilson  of  Glasgow 
first  propounded  his  well-known  theory,  that  the  fore- 
shortening of  the  penumbral  filaments  as  the  spot  neared 
the  sun's  edge  showed  that  the  spots  were  saucer-like  de- 
pressions in  the  sun's  general  surface.  Spots  having  been 
seen  which  confirmed  the  theory  while  others  seemed  to  dis- 
prove it,  the  arguments  have  gone  on  pro  and  con.  For  a 
more  complete  discussion,  see  Agnes  M.  Clerke,  Problems  in 
Astrophysics.  In  view  of  the  very  rapid  change  in  gravita- 
tion near  the  surface  of  the  sun  already  noted,  it  is  alto- 
gether probable  that  the  elevation  at  which  sun-spots  exist 
differs  less  than  fifty  kilometers  from  that  of  the  photo- 
sphere. This  small  allowable  difference  of  altitude  is  not 
sufficient  to  permit  the  saucer-like  sinks  needed  for  Wilson's 
theory. 

The  most  evident  fact  concerning  the  spots  is  their  peri- 
odicity, first  discovered  in  1843  by  Schwabe,  the  average 
period  being  11.13  years.  There  are  marked  differences  in 
the  length  from  maximum  to  maximum,  and  equally  great 
divergences  in  the  intensity  of  the  various  maxima  so  that 
it  must  be  said  that  spots  are  very  irregular  in  their  regu- 
larity. The  individual  periods  range  between  7.3  and  17.1 
years  as  extremes.  But  no  matter  how  divergent  the  period 
is  from  the  mean,  the  rise  to  maximum  spottedness  always 
consumes  less  time  than  the  descent  to  minimum.  On  the 
average  the  intervals  are  4.62  years  and  6.51  years,  respec- 
tively. The  sun-spot  curve  thus  resembles  the  light  curve 
of  the  average  variable  star  of  long  period,  and  also  those 
of  the  Cepheids.  The  importance  of  this  fact  is  here 


120  ECLIPSES    OF    THE    SUN 

emphasized  for  the  reason  that  we  have  learned  from  the 
researches  of  Abbot  that  the  total  radiation  of  the  sun  varies 
in  amount,  and  as  a  consequence  the  sun  must  be  regarded 
as  a  variable  star  of  long  period. 

Various  attempts  have  been  made  to  examine  the  sun-spot 
curve  by  the  methods  of  harmonic  analysis  in  order  to  find 
any  secondary  periods  that  may  underlie  the  main  period 
of  11.13  years.  The  most  notable  attempts  in  recent  years 
have  been  by  Schuster/  Hirayama,2  Kimura,3  Michelson,4 
and  Larmor  and  Yamaga.5  Kimura  and  Michelson  each 
examined  160  years  of  sun-spot  records  from  1750  to  1910, 
and  although  the  material  for  examination  was  the  same,  the 
conclusions  reached  are  greatly  at  variance.  The  former 
decides,  "  The  n-year  period  is  not  so  conspicuous  as  gen- 
erally considered.  Although  the  most  important  of  all,  yet 
to  my  surprise  there  are  a  great  many  periodicities  lying  be- 
tween 8  and  12  years,  most  of  them  being  of  considerable 
relative  amplitude. "  Kimura  predicts  the  form  of  the  sun- 
spot  curve  up  to  the  year  1950,  the  predictions  giving  a  maxi- 
mum of  spots  at  the  beginning  of  the  year  1914,  with  an 
intensity  about  equivalent  to  that  of  the  maximum  of 
iqos-6.  According  to  observations  at  Mt.  Wilson,  however, 
Nicholson G  finds  that  the  maximum  did  not  take  place 
until  August,  1917,  over  three  years  later  than  the  pre- 
dicted time,  while  the  activity  was  considerably  greater 
than  at  the  preceding  maximum.  Michelson  concludes 
that  "with  the  exception  of  the  n-year  period  and  pos- 
sibly a  very  long  period  (of  the  order  of  100  years), 
the  many  periods  found  by  previous  investigations  are  illu- 
sory. "  Quite  similar  deductions  are  drawn  by  Larmor  and 
Yamaga  who  even  go  so  far  as  to  state  that  when  the  peri- 
odic part  is  removed  the  residue  of  the  sun-spot  activity  is 
of  a  fortuitous  sporadic  character,  not  amenable  to  further 
analysis.  Various  attempts  have  likewise  been  made  to  ex- 
plain the  sun-spots  by  means  of  the  attractions  of  the 

1  Phil.  Trans.  Roy.  Soc.  206. 

2  Tokyo  Sugata,  3,  g. 

3  M.  N.  R.  A.  S.,  73,  543- 

4  Astro  physical  Journal,  38,  268. 

5  Royal  Soc.  Proc.  A.,  93,  493. 
0  Publ.  A.  S.  P.,  Ji,  223,  1919. 


THE    SURFACE    OF    THE    SUN  121 

planets,  particularly  those  of  the  giant  of  the  sun's  family. 
But  Jupiter's  period  is  n.86  years,  and  even  when  other 
periods  of  other  planets  are  superimposed,  no  success  has 
followed  the  attempts  to  explain  sun-spots  by  planetary 
influences.  The  cause  of  the  cycle  indeed  seems  as  much 
unknown  today  as  when  a  hundred  years  ago  Schwabe  first 
began  his  systematic  observations. 

Spots  are  never  found  more  than  45°  from  the  equator 
and  seldom  at  the  equator.  A  curious  distribution  of  the 
spots  in  latitude  manifests  itself  during  the  progress  of  the 
solar  cycle.  Approximately  at  the  time  of  minimum,  sun- 
spots  begin  to  manifest  themselves  in  two  zones,  more  than 
30°  north  and  south  of  the  equator.  With  the  lapse  of  time, 
the  spots  are  found  closer  and  closer  to  the  equator,  the 
sun-spot  maximum  taking  place  with  the  spots  in  zones  17° 
north  and  south.  The  disturbance  gradually  dies  out  in 
latitude  8°  or  10°,  after  a  lapse  of  13  or  14  years  from  the 
first  outbreak.  Before  the  final  flickering  out,  the  new  cycle 
has  begun  to  manifest  itself,  so  that  near  sun-spot  minimum 
there  are  found  four  zones  of  disturbance,  two  near  the 
equator,  and  two  farther  north  and  south. 

Many  serious  attempts  have  been  made  to  connect  sun- 
spots  with  various  phenomena,  solar  and  also  terrestrial. 
The  correlation  with  the  solar  manifestations  mainly  rests 
on  secure  ground,  but  with  some  of  the  earthly  influences, 
the  connection  seems  rather  far-fetched.  If  it  is  hotter  than 
the  average  at  a  certain  locality  in  the  United  States,  like 
St.  Louis,  or  if  mayhap  at  the  same  time,  Northern  France  is 
having  a  cold  spell,  and  if  coincidently  there  is  a  large  spot- 
group  on  the  sun,  an  astronomer,  or  usually  pseudo-astrono- 
mer, is  always  found  who  informs  the  daily  press  that  the 
sun-spot  is  the  cause  of  the  heat  (or  cold).  The  famines  in 
India,  the  potato  crop  in  Ireland,  the  price  of  corn  in  Eng- 
land, the  rain- fall  in  the  Island  of  Mauritius,  the  financial 
panics  of  Wall  Street  all  have  been  investigated  by  statistical 
methods,  and  each  and  all  have  been  found  to  pass  through 
periods  of  the  same  length,  and  to  be  connected  with  the  sun- 
spot  period.  It  has  many  times  been  said  that  "  figures  never 
lie."  It  is  quite  true  that  the  figures  themselves  do  not  tell 


122  ECLIPSES    OF    THE    SUN 

falsehoods,  but  many  and  varied  are  the  interpretations  that 
may  be  placed  on  these  figures.  The  president  of  every  big 
business  corporation  well  knows  that  if  his  company  shows 
in  two  successive  years  the  same  approximate  amount  of 
earnings,  it  is  very  easy  for  him  to  declare  a  substantial 
dividend,  or  to  charge  certain  amounts  to  "  improvements/' 
and  curtail  or  even  pass  a  dividend.  The  whole  question 
depends  on  whether  he  wishes  to  please  the  fifty-one  percent 
of  the  majority  stock-holders  (of  whom  he  is  one),  or  to 
take  advantage  of  the  forty-nine  percent  (of  whom  he  is  not 
one).  It  is  quite  possible,  and  indeed  probable,  that  the 
weather  and  rainfall  are  connected  with  variations  of  solar 
activity  as  evidenced  by  sun-spots,  and  that  other  manifesta- 
tions of  meteorological  changes  are  also  correlated, — but  to 
prove  the  connection  "  is  another  story."  The  weather  is 
"  not  made  on  the  spot/'  it  depends  on  a  vast  variety  of 
conditions  and  it  is  therefore  difficult  and  well-nigh  im- 
possible to  single  out  the  solar  cause  from  all  of  the  possible 
influences  that  may  affect  the  weather.  It  might  not  be  out 
of  place  to  remind  all  such  investigators  that  a  great  variety 
of  periods  have  been  found  in  the  sun-spot  cycle  itself,  but 
that  none  of  the  subsidiary  periods  appear  to  have  any 
reality,  no  matter  how  firmly  substantiated  by  figures  they 
seem  to  be.  Most  of  the  meteorological  dependences  seem 
to  be  equally  illusory.  There  are,  however,  many  terrestrial 
and  many  solar  phenomena  which  have  a  well  proven  con- 
nection with  sun-spots.  These  will  be  given  below  in  brief 
form,  though  some  of  them  will  later  be  expanded  more 
fully. 

Records  kept  at  the  Greenwich  Observatory  and  extend- 
ing over  nearly  a  hundred  years,  show  (i),  that  the  diurnal 
range  of  the  magnetic  declination,  and  (2),  that  the  hori- 
zontal force  of  the  magnetism  flowing  through  the  earth, 
follow  the  sun-spot  fluctuations  not  only  in  the  main  n-year 
period  but  even  in  the  small  and  secondary  variations.  The 
parallelism  is  so  intimate  that  it  is  at  once  evident  that  if 
the  cause  can  be  found  of  the  sun-spot  cycle  there  also  will 
be  found  the  true  explanation  of  the  variation  of  terrestrial 
magnetism.  Although  the  records  are  not  so  complete,  (3), 


THE    SURFACE    OF    THE    SUN  123 

aurorae  and  (4),  magnetic  storms  are  more  abundant  when 
spots  are  numerous.  The  same  may  be  said  (5),  of  faculae, 
and  (6),  of  prominences.  (7)  The  shape  of  the  corona 
changes  with  the  sun-spot  period;  at  minimum  of  spots  there 
are  long  equatorial  extensions,  and  well  defined  polar  rays. 
(8)  The  conclusions  of  Koppen,  Stone,  Gould,  Nordmann, 
Newcomb,  Abbot  and  Fowle,  Arctowski  and  Bigelow  are 
that  there  is  a  change  in  the  mean  temperature  of  the  earth, 
small  in  size,  but  amounting  to  0.7°  between  sun-spot  maxi- 
mum and  minimum,  the  earth  being  cooler  at  sun-spot 
maximum,  which,  in  fact,  is  quite  contrary  to  the  ordinary 
popular  belief. 

The  contribution  of  the  spectroscope  regarding  spots  and 
related  phenomena  will  be  given  in  a  subsequent  chapter. 

The  state  of  our  knowledge  of  the  angular  diameter  of 
the  sun  is  far  from  satisfactory.  The  accepted  value  of  this 
fundamental  quantity  comes  from  measurements  made  many 
years  ago  with  the  heliometer.  The  discussion  by  Schurr 
and  Ambronn  gave  the  angular  diameter  of  the  sun  when 
at  its  mean  distance  from  the  earth  to  be  equal  to  1 920^.0 
=*=  o".c>3.  The  investigations  showed  that  there  were  slight 
but  unmistakable  differences  between  the  polar  and  equa- 
torial diameters,  but  in  spite  of  these  differences  it  was  as- 
sumed that  the  sun  was  spherical.  In  this  procedure  they 
followed  the  example  set  by  the  great  master,  Auwers. 

In  the  present  day  of  refinement  in  solar  research  it  seems 
quite  unsafe  to  assume  that  the  sun  is  necessarily  spherical 
or  that  the  changes  taking  place  in  the  polar  and  equatorial 
diameters  are  so  small  that  they  are  beyond  the  possibility 
of  measurement.  The  heliometer,  as  the  name  signifies,  was 
invented  for  the  purpose  of  measuring  the  diameter  of  the 
sun,  the  first  heliometer  coming  from  the  hands  of  Fraun- 
hofer  (p.  77).  It  is  a  very  valuable  and  refined  instrument 
of  measurement,  and  in  addition  to  its  use  on  the  sun  it  has 
been  extensively  employed  in  the  determination  of  stellar 
parallaxes.  The  latter  research  has  shown  its  limitations. 
When  used  on  the  sun,  the  great  heat  of  the  sun  causes  the 
same  effect  which  is  found  in  every  instrument  of  precision 
when  exposed  to  the  sun's  rays,  namely,  the  instrument  is 


124  ECLIPSES    OF    THE    SUN 

put  out  of  accurate  adjustment.  The  heliometer  being 
specially  sensitive  to  changes  in  adjustment  it  is  not  sur- 
prising to  find,  by  reference  to  the  original  observations, 
that  the  same  observer,  using  the  same  instrument  on  two 
successive  days  and  under  good  observing  conditions,  would 
obtain  differences  in  the  measurement  of  the  equatorial  di- 
ameter of  the  sun  amounting  to  i",  2",  5",  or  even  10"  or 
more.  In  addition,  every  observer  has  a  "  personal  equa- 
tion "  in  that  he  may  constantly  measure  a  quantity  smaller 
or  larger  than  the  average  observer.  Each  astronomer  who 
has  made  visual  or  photographic  measures  is  familiar  with 
the  problem  of  systematic  errors.  With  the  sun  it  is  difficult 
to  know  how  to  allow  for  irradiation  which  causes  a  spread- 
ing of  the  image. 

The  only  practicable  method  of  combining  observations 
inconsistent  among  themselves  is  to  group  all  the  measures 
together  and  take  the  mean,  at  times  assigning  different 
weights.  If  the  quantity  of  observations  is  sufficiently  great 
and  the  number  of  observers  numerous  enough,  it  is  quite 
safe  to  assume  that  the  peculiarities  of  any  one  individual 
will  have  little  effect  in  the  final  mean.  For  the  determina- 
tion of  the  diameter  of  the  sun,  Auwers  had  at  his  disposal 
no  less  than  15,000  observations  made  by  100  observers 
working  between  the  years  1851  and  1883.  The  measures, 
however,  were  not  all  made  by  the  heliometer. 

The  large  systematic  errors  to  which  these  measures  were 
subject  have  had  a  curious  effect  on  future  observations,  a 
parallel  to  which  is  not  found  in  any  other  department  of 
astronomical  investigation.  The  result  has  virtually  been  to 
terminate  all  observational  measures  of  the  solar  diameter. 
What  would  it  avail  any  astronomer  if  he  should  measure  the 
diameter  of  the  sun  with  the  heliometer  on  hundreds  of  days, 
spending  perhaps  many  thousands  of  hours  of  diligent  toil 
in  the  research,  only  to  find  that  his  results  differed  from  the 
generally  accepted  value?  To  the  astronomical  world  this 
difference  would  probably  be  regarded  as  a  proof,  not  that 
the  accepted  value  was  in  error  or  that  the  diameter  of  the 
sun  was  changing,  but  rather  that  the  measures  of  the  in- 
dividual though  made  with  the  greatest  of  refinement  were 


THE    SURFACE    OF    THE    SUN  125 

subject  to  personal  equations.  At  best  the  astronomer  se- 
cures little  reward  for  the  hard  toil  devoted  to  his  skilled 
researches.  Moreover,  he  is  a  human  being  and  naturally 
he  desires  some  compensation  other  than  that  of  advertising 
to  the  rest  of  the  scientific  world  that  he  is  a  faulty  ob- 
server. And  this,  strange  to  relate,  is  the  state  of  affairs  in 
the  enlightened  days  of  the  twentieth  century  when  hun- 
dreds of  thousands  of  dollars  are  spent  each  year  in  solar 
research !  Astronomy  appears  to  be  virtually  saying  that  no 
improvements  are  possible  in  the  work  done  nearly  half  a 
century  ago  by  the  heliometer  —  and  so  we  shall  assume 
that  the  sun  is  spherical  and  without  change. 

Are  no  other  methods  available?  Why  not  try  photog- 
raphy? Surely  the  great  resources  of  modern  astronomy 
can  conquer  any  difficulties !  There  are  indeed  no  difficulties 
of  any  note  connected  with  the  photographic  processes,  for 
excellent  photographs  of  the  sun  are  being  secured  daily. 
The  main  difficulty  to  be  overcome  is  the  same  one  that 
affects  heliometer  work,  namely,  the  heat  of  the  sun.  This 
alters  the  length  of  the  telescope  tube  and  changes  the  focal 
length  of  the  object  glass  so  that  the  exact  scale  of  the  photo- 
graphs is  uncertain.  These  changes,  however,  do  not  alter 
the  relative  scales  of  the  polar  and  equatorial  diameters. 
Some  indefatigable  worker,  therefore,  has  already  waiting 
to  his  hand  some  hundreds  of  thousands  of  solar  negatives 
to  be  measured  and  discussed  for  the  purpose  of  determining 
whether  or  not  the  sun  is  spherical. 

The  only  method  apparently  available  of  eliminating  the 
effect  of  the  heat  of  the  sun  and  at  the  same  time  applying 
photography  is  clearly  outlined  by  Hayn  1  who  applied  it 
with  great  success  at  the  eclipses  of  April  17,  1912,  and 
August  21,  1914.  By  means  of  photographs  taken  at  the 
time  of  a  solar  eclipse,  not  however,  during  totality,  but 
during  the  partial  phases,  the  shape  and  size  of  the  sun  can 
be  determined,  the  shape  and  size  of  the  moon  also,  and  in 
addition,  the  times  of  contacts  of  the  limbs  of  the  sun  and 
moon  usually  secured  at  eclipses. 

A  research  somewhat  similar  in  character  has  been  carried 

1  Astronomische  Nachrichten,  201,  185,  1915. 


126  ECLIPSES    OF    THE    SUN 

out  by  Henry  Norris  Russell *  who  measured  photographs 
taken  at  Harvard  College  Observatory  for  the  purpose  of 
determining  the  position  of  the  moon  with  respect  to  the 
stars.    To  secure  satisfactory  photographs,  it  was  necessary 
to  make  the  exposures  on  the  moon  the  thousandth  part  of 
those  required  for  the  stars,  and  at  the  same  time  the  tele- 
scopic object-glass  had  to  be  shielded  from  the  light  of  the 
moon  so  that  the  photographic  plate  might  not  be  fogged. 
The  difficulties  to  be  surmounted  by  Hayn's  method  during 
the  progress  of  the  eclipse  are  not  as  great  as  those  overcome 
at  Harvard  in  thus  photographing  the  moon.     To  give  in- 
formation of  the  highest  degree  of  reliability,  it  is  necessary 
to  know  the  latitude  and  longitude  and  the  observed  times 
with  great  precision.    On  this  account  it  would  be  preferable 
to  test  Hayn's  method  at  a  fixed  observatory  rather  than 
to  attempt  it  under  the  temporary  conditions  of  an  eclipse 
expedition.     The  best  locations  will  be  those  nearest  the 
path  of  totality.    At  the  eclipse  of  September  10,  1923,  three 
great  American  observatories  and   the  Mexican  National 
Observatory  at  Tacubaya  were  conveniently  located,  the 
American  institutions  being  Mt.  Wilson,  Lick  and  Lowell 
observatories.    With  the  superb  instrumental  equipment  of 
the  Mt.  Wilson  Observatory  and  an  eclipse  ninety-eight 
percent  total,  what  magnificent  scientific  results  might  not 
have  been  obtained  if  the  skies  had  not  been  densely  cloudy! 
At  the  total  eclipse  of  January  24,  1925,  the  Van  Vleck 
Observatory  with  its  2o-inch  visual  refractor  was  in  the  path 
of  totality,  and  consequently  a  series  of  photographs  were 
taken  between  third  and  fourth  contacts.     On  account  of 
the  poor  conditions  of  seeing,  everywhere  observed  at  this 
eclipse,  the  limbs  of  both  sun  and  moon  were  not  sharp. 
Measures  2  of  ten  Van  Vleck  photographs  and  two  made  with 
the  Yale  refractor  show  the  difficulties  of  making  proper  al- 
lowance for  irradiation,  or  spreading,  of  the  photographic 
images.    It  might  be  possible  to  allow  for  uniform  irradiation 
but  it  is  more  difficult  to  take  care  of  differential  irradiation 
due  to  the  fact  that  the  intensity  of  the  solar  radiation  de- 

1  Harvard  Annals,  72,  76  and  80. 

2  Astronomical  Journal,  37,  32,  33,  1926. 


THE    SURFACE    OF    THE    SUN  127 

creases  toward  the  limb,  thus  greatly  affecting  plates  taken 
when  the  solar  cusp  is  small  near  the  time  of  totality.  Evi- 
dently the  only  practicable  method  would  be  to  take  photo- 
graphs both  before  and  after  totality. 

At  the  brief  eclipse  of  April  28,  1930,  three  photographs 
were  taken  '  with  the  36-inch  Lick  refractor,  in  spite  of  wide- 
spread clouds. 

The  eclipse  of  October  21,  1930  gave  the  opportunity  of 
taking  sixteen  photographs  with  each  of  two  cameras,  one 
38-inches,  the  other  i5-feet  in  focal  length.  The  lenses  were 
fitted  with  graflex  shutters  and  were  stopped  down.  With 
each  camera,  eight  plates  were  taken  at  one-minute  intervals 
starting  shortly  after  first  contact,  and  another  eight  plates 
ending  shortly  before  fourth  contact.  The  plates  were  used 
to  determine  the  times  of  first  and  fourth  contacts.  Differ- 
ences were  found  in  the  intensities  of  the  plates  caused  by 
variations  in  transparency  and  alterations  in  the  rate  of  the 
exposing  shutter.  The  plates  were  measured  2  independently 
by  Marriott  and  Pitman.  The  mean  of  their  measures  shows 
that  the  1 5-foot  camera  gave  the  time  of  first  contact  1.2  sec- 
onds earlier,  and  the  time  of  fourth  contact  1.2  seconds  later, 
than  was  obtained  from  the  38-inch  camera;  an  effect  un- 
questionably due  to  irradiation.  Assuming  a  correction  of 
+  i ".50  to  the  sun's  mean  longitude,  the  photographs  showed 
that  the  American  Ephemeris  position  of  the  moon  needed 
corrections  of  -j-  5". 54  in  mean  longitude  and  — 0^.25  in 
latitude.  For  predicting  the  eclipse,  Washington  had  used  a 
correction  to  the  moon's  longitude  of  +  5"-47  obtained  from 
occultations.  The  photographs  showed  that  first  contact  was 
1.5  seconds  earlier  and  fourth  contact  o.i  seconds  later  than 
the  predicted  times. 

1  Publ.  A.  S.  P.,  42,  i45,  1930. 

2  Astronomical  Journal,  41,  129,  1931. 


CHAPTER   VIII 

MODERN   ECLIPSES    BEFORE    1878 

"Tycho  sought  the  truth 

From  that  strange  year  in  boyhood  when  he  heard 
The  great  eclipse  foretold;  and,  on  the  day 
Appointed,  at  the  very  minute  even, 
Beheld  the  weirdly  punctual  shadow  creep 
Across  the  sun,  bewildering  all  the  birds 
With  thoughts  of  evening."  —  NOYES. 

ASTRONOMY  owes  much  to  the  eclipse  of  the  sun 
visible  in  Copenhagen  on  August  21,  1560,  and  to 
the  fact  that  a  red-headed,  freckled-face  boy  of 
fourteen ,  destined  to  become  the  greatest  and  most  careful 
of  observational  astronomers  since  the  time  of  Hipparchus, 
had  his  keen  young  imagination  fired  by  watching  the 
"  orange  ember  in  the  sky  wane  into  smouldering  ash.'7  As 
a  consequence,  this  boy,  Tycho  Brahe,  resolved  to  devote 
his  life  to  unravelling  the  deep  mystery  of  these  strange 
happenings.  The  romantic  incidents  of  his  productive  life 
have  been  beautifully  told  by  Alfred  Noyes.  From  his  own 
printing  press  in  his  observatory  of  Uranibourg  appeared 
the  Historia  Coelestis  in  which  appears  a  long  list  of  eclipses 
beginning  with  one  visible  in  Rome  on  March  28,  in  the 
year  5  A.D. 

At  the  eclipse  of  May  3,  1715,  Halley  referred  to  that  of 
the  year  1140  as  the  last  one  previously  observed  in  London. 
Although  visible  not  far  from  London,  Hind  finds  from  in- 
vestigation that  this  eclipse  was  not  seen  in  the  city  itself, 
so  that  it  can  be  said  with  certainty  that  not  a  single  total 
eclipse  of  the  sun  had  visited  London  for  600  years  previous 
to  1715. 

The  first  eclipse  of  the  sun  to  be  carefully  observed  in 
the  British  Colonies  of  America  was  that  of  June  24,  1778, 
which  was  watched  by  the  astronomer  David  Rittenhouse 

128 


DETAILS  OF  THE  HYDROGEN   PROMINENCES,   JUNE  8,   1918,   INCLUDING   THE   "  EAGLE 

PROMINENCE  " 

From  the  painting  by  Howard  Russell  Butler,  N.  A. 


MODERN    ECLIPSES    BEFORE    1878  129 

of  Philadelphia.  The  first  American  expedition  was  orga- 
nized and  sent  out  from  Harvard  College  for  the  eclipse  of 
October  27,  1780.  As  this  took  place  during  the  war  of  the 
American  Revolution,  an  appeal  was  made  to  "  the  govern- 
ment of  the  Commonwealth  that  a  vessel  might  be  prepared 
to  convey  proper  observers  to  Penobscot-Bay;  and  that  ap- 
plication might  be  made  to  the  officer  who  commanded  the 
British  garrison  there,  for  leave  to  take  a  situation  con- 
venient for  this  purpose. 

"  Though  involved  in  all  the  calamities  and  distresses  of 
a  severe  war,  the  government  discovered  all  the  attention 
and  readiness  to  promote  the  cause  of  science,  which  could 
have  been  expected  in  the  most  peaceable  and  prosperous 
times;  and  passed  a  resolve,  directing  the  Board  of  War  to 
fit  out  the  Lincoln  galley  to  convey  me  to  Penobscot,  or  any 
other  port  at  the  eastward,  with  such  assistants  as  I  should 
judge  necessary. 

"  Accordingly,  I  embarked  October  9."  * 

Probably  on  account  of  an  error  in  the  tables,  the  eclipse 
was  not  total  where  the  Harvard  party  was  located.  Be- 
tween the  first  and  second  contacts  Professor  Williams 
measured  the  angular  length  of  the  moon  subtended  by  the 
decreasing  crescent  of  the  sun.  He  gives  the  following  de- 
scription of  what  appeared  shortly  before  the  total  phase 
was  expected:  "  The  sun's  limb  became  so  small  as  to  ap- 
pear like  a  circular  thread  or  rather  like  a  very  fine  horn. 
Both  the  ends  lost  their  acuteness  and  seemed  to  break  off 
in  the  form  of  small  drops  or  stars  some  of  which  were  round 
and  others  of  an  oblong  figure.  They  would  separate  to  a 
small  distance,  some  would  appear  to  run  together  again 
and  then  diminish  until  the  whole  disappeared." 

Apparently  this  is  a  clear  description  of  the  so-called 
"  Baily's  Beads  "  observed  by  Francis  Baily  at  the  eclipse 
of  1836.  An  excellent  description  of  this  phenomenon  is 
given  by  Agnes  Clerke  in  her  History  of  Astronomy  during 
the  Nineteenth  Century,  page  74.  Baily  gave  the  correct 
explanation  of  the  phenomenon  he  saw  as  being  due  to  ir- 
radiation. This  same  effect  is  seen  when  one  holds  up  his 

1  Memoirs  American  Academy  of  Arts  and  Sciences,  /,  84,  1783. 


130  ECLIPSES    OF    THE    SUN 

hand  to  the  sunlight.  In  making  the  fingers  come  close  to- 
gether, they  appear  to  touch  each  other  before  one  feels 
they  are  actually  in  contact.  An  analogous  manifestation 
is  called  the  "  black  drop  "  which  caused  surprise  at  the 
transits  of  Venus  in  the  years  1761  and  1769,  and  was  the 
source  of  great  trouble  to  astronomers  at  the  transits  of  1874 
and  1882,  so  widely  observed  for  the  purpose  of  determining 
the  solar  parallax.  The  appearance  of  "  Baily's  Beads  "  is 
a  phenomenon  well  worth  watching  and  should  be  atten- 
tively looked  for  just  before  totality  begins  and  just  after 
it  ends.  Very  excellent  observations  may  be  made  with  a 
good  pair  of  field  glasses  or  with  a  small  telescope,  a  large 
telescope  being  unnecessary. 

Baily  was  not  an  astronomer  by  profession.  He  was  a 
stock-broker,  and  fortunately  he  had  been  successful  in  the 
making  of  money,  with  the  result  that  he  was  able  to  devote 
the  maturer  years  of  his  life  to  astronomy  which  he  took  up 
as  his  hobby.  His  work  is  but  one  of  the  many  instances 
of  the  great  debt  of  science  to  the  amateur  astronomer. 
One  important  result  of  his  observations  in  1836  was  to 
show  professional  astronomers  that  at  the  time  of  the  total 
eclipse  of  the  sun  there  were  other  phenomena  to  observe 
than  the  mere  times  of  contact  of  the  limbs  of  the  sun  and 
moon. 

The  eclipse  of  1836  witnessed  not  only  the  phenomenon 
of  "  Baily's  Beads  "  but  also  an  attempt  by  Forbes  to  test 
the  physical  constitution  of  the  sun's  atmosphere  by  means 
of  the  spectroscope.  A  new  era  for  astronomy  had  accord- 
ingly dawned.  An  eclipse  occurred  in  Southern  Europe  on 
July  8,  1842,  and  into  the  narrow  track  were  collected  the 
foremost  astronomers  from  England,  France,  Germany  and 
Russia.  What  was  observed  in  1836  was  as  nothing  com- 
pared with  the  wonders  of  the  eclipse  of  1842! 

One  of  the  strangest  portions  of  the  history  of  astronomy 
before  the  middle  of  the  nineteenth  century  is  the  evident 
lack  of  interest  in,  or  perhaps  one  should  say,  the  dearth 
of  accurate  observations  of  the  phenomena  visible  at 
the  time  of  a  total  eclipse  of  the  sun.  The  startling 
suddenness  of  the  apparition,  coming  in  the  early  days  of 


MODERN    ECLIPSES    BEFORE    1878  131 

civilization  without  warning,  must  have  brought  terror  to 
the  hearts  of  the  populace  and  caused  them  to  fear 
war  or  pestilence  or  the  death  of  a  favorite  prince. 
It  is  but  natural  that  the  prehistoric  superstition  of 
the  dragon  swallowing  the  sun  should  have  spread  during 
the  middle  ages  from  the  far  East  to  all  of  the  civilized 
world.  One  fleeting  glance ,  however,  should  have  revealed, 
even  to  the  most  timorous  minded,  the  pearly-gray  light  of 
the  corona  and  brought  to  view  the  glow  of  the  rosy-hued 
prominences.  To  those  of  the  present  generation,  the  nine 
hundred  and  ninety-nine  out  of  a  thousand  who  have  never 
had  the  good  fortune  to  witness  a  total  solar  eclipse,  it  might 
not  be  out  of  place  to  point  out  that  a  telescope  is  entirely 
unnecessary  for  viewing  the  beauties  of  the  corona,  this 
being  a  spectacle  that  derives  its  glory  from  the  wide-spread 
splendor  and  slight  gradations  of  contrast.  A  telescope, 
small  or  large  will  of  course  magnify  any  particular  portion 
—  but  to  see  and  enjoy  the  beauty  of  the  corona  as  a  whole 
nothing  is  actually  needed  but  the  normal  naked  eye. 

Published  references  to  the  corona  in  the  early  literature 
are  exceedingly  rare.  Plutarch  and  Philostratus  give  allu- 
sions which  unmistakably  refer  to  the  corona,  but  appar- 
ently the  first  to  take  any  scientific  cognizance  of  the  crown 
of  glory  was  Kepler  who  seems  to  have  witnessed  the  solar 
eclipse  of  1605  in  Naples.  A  hundred  years  later  at  the 
eclipse  of  1706,  Cassini,  who  was  a  practised  observer,  de- 
scribes the  "  crown  "  of  pale  light,  and  he  decides  that  it 
must  be  caused  by  the  illumination  of  zodiacal  light;  and 
eleven  years  thereafter,  Halley  saw  the  corona  and  also 
prominences,  but  he  was  unable  to  decide  whether  the  corona 
belonged  to  the  sun  or  to  the  moon. 

If  so  little  attention  was  paid  to  the  corona,  it  is  not  sur- 
prising that  even  less  should  be  taken  of  the  "  red  flames/' 
though  if  one  refers  to  the  plate  facing  page  130  he  will  see 
what  a  brilliant  spectacle  they  afforded  in  the  eclipse  of 
1918.  The  first  reference  to  them  seems  to  be  at  the  eclipse 
of  1706  when  they  were  apparently  observed  by  Stannyan 
who  wrote  a  description  of  them  to  Flamsteed.  The  first 
vivid  portrayal  was  by  Vassinius  of  Sweden  who  observed 


132  ECLIPSES    OF    THE    SUN 

them  in  1733.  The  Spanish  admiral  Ulloa  observed  them 
while  at  sea  during  the  eclipse  of  June  24,  1778,  and  he 
furnished  a  valuable  account,  with  the  added  explanation 
that  the  rosy  hues  were  caused  by  the  sun's  light  shining 
through  some  hole  or  crevice  in  the  limb  of  the  moon! 

The  astronomers  who  witnessed  the  eclipse  of  1842  were 
entirely  unprepared  for  the  phenomena  that  met  their  gaze. 
Baily  repaired  to  Pavia,  and  made  his  observations  from 
one  of  the  rooms  of  the  University.  One  of  the  professors, 
out  of  the  goodness  of  his  heart,  offered  to  assist  him  in  any 
way  possible,  but  Baily  informed  him  that  all  he  wanted 
was  to  be  "  left  alone,  being  persuaded  that  nothing  is  so  in- 
jurious to  the  making  of  accurate  observations,  as  the  intru- 
sion of  unnecessary  company. "  Not  being  content  with  this 
gentle  hint,  the  key  was  taken  from  the  outside  of  the  door 
and  it  was  securely  locked  on  the  inside.  Baily's  report  of 
the  observations  made  at  the  eclipse  is  found  in  the  Memoirs, 
R.  A.  S.,  15,  4,  1846,  as  follows:  "  The  beads  were  distinctly 
visible.  ...  I  was  astounded  by  a  tremendous  burst  of  ap- 
plause from  the  streets  below,  and  at  the  same  moment  was 
electrified  at  the  sight  of  one  of  the  most  brilliant  and  splen- 
did phenomena  that  can  be  imagined.  For  at  that  instant 
the  dark  body  of  the  moon  was  suddenly  surrounded  with 
a  corona,  or  kind  of  bright  glory.  ...  I  had  indeed  antici- 
pated a  luminous  circle  round  the  moon  during  the  time  of 
total  obscurity,  but  I  did  not  expect,  from  any  of  the  ac- 
counts of  previous  eclipses  that  I  had  read,  to  witness  so 
magnificent  an  exhibition  as  that  which  took  place.  .  .  . 
The  breadth  of  the  corona,  measured  from  the  circumference 
of  the  moon,  appeared  to  me  to  be  nearly  equal  to  half  the 
moon's  diameter.  It  had  the  appearance  of  brilliant  rays. 
Its  colour  was  quite  white,  not  pearl  colour,  nor  yellow, 
nor  red. 

"  Splendid  and  astonishing,  however,  as  this  remarkable 
phenomenon  really  was,  and  although  it  could  not  fail  to 
call  forth  the  admiration  and  applause  of  every  beholder, 
yet  I  must  confess  that  there  was  at  the  same  time  some- 
thing in  its  singular  and  wonderful  appearance  that  was 
appalling.  .  ,  ,  But  the  most  remarkable  circumstance  at- 


MODERN    ECLIPSES    BEFORE    1878  133 

tending  this  phenomenon  was  the  appearance  of  three  large 
protuberances  apparently  emanating  from  the  circumference 
of  the  moon,  but  evidently  forming  a  portion  of  the  corona. 
.  .  .  All  of  these  projections  were  of  the  same  roseate  cast 
of  colour,  and  very  distinct  from  the  brilliant  vivid  white 
light  that  formed  the  corona.  .  .  .  The  whole  of  these  three 
protuberances  were  visible  even  to  the  last  moment  of  total 
obscuration,  at  least,  I  never  lost  sight  of  them  when  look- 
ing in  that  direction;  and  when  the  first  ray  of  light  was 
admitted  from  the  sun,  they  vanished  with  the  corona,  alto- 
gether, and  day-light  was  instantly  restored." 

The  same  appearance  was  witnessed  by  Airy,  by  Arago 
and  others.  Arago  has  an  interesting  account  of  the  effect 
of  the  eclipse  on  the  populace  who  had  gathered  in  great 
numbers  to  watch  the  phenomenon.  "  When  the  sun,  re- 
duced to  a  very  narrow  filament,  began  to  throw  upon  the 
horizon  only  a  very  feeble  light,  a  sort  of  uneasiness  seized 
upon  all;  every  one  felt  a  desire  to  communicate  his  impres- 
sions to  those  around  him.  Hence  arose  a  deep  murmur, 
resembling  that  sent  forth  by  the  distant  ocean  after  a 
tempest.  The  hum  of  voices  increased  in  intensity  as  the 
solar  crescent  grew  more  slender;  at  length  the  crescent 
disappeared  and  an  absolute  silence  marked  this  phase  of 
the  eclipse.  The  phenomenon  in  its  magnificence  had  tri- 
umphed over  the  petulance  of  youth,  over  the  levity  which 
certain  persons  assume  as  a  sign  of  superiority,  over  the 
noisy  indifference  of  which  soldiers  usually  make  profession. 
A  profound  stillness  also  reigned  in  the  air,  the  birds  had 
ceased  to  sing." 

The  arrival  of  totality  in  Milan  was  greeted  by  a  great 
shout,  mingled  with  cries  of  "  Long  live  the  astronomers  " 
who  had  provided  such  a  beautiful  phenomenon  to  please 
and  interest  the  populace! 

The  unexpected  nature  of  prominences  and  corona  seen 
at  the  eclipse  coupled  with  the  publication  in  1843  of 
Schwabe's  discovery  of  the  periodicity  of  sun-spots  caused 
an  unprecedented  increase  of  interest  in  matters  pertaining 
to  the  physical  constitution  of  the  sun.  Various  ingenious 
explanations  appeared.  While  most  astronomers  believed 


134  ECLIPSES    OF    THE    SUN 

that  the  prominences  were  truly  solar  in  their  origin,  there 
were  many  who  thought  they  were  possibly  some  exhalation 
in  the  earth's  upper  atmosphere,  while  still  others  believed 
that  in  some  manner  diffraction  round  the  edge  of  the  moon 
was  responsible  for  these  eclipse  envelopes.  In  passing,  we 
might  mention  a  notion  of  no  less  an  authority  than  Halley 
which  was  so  curious  that  it  should  be  classed  along  with 
William  HerschePs  belief  that  the  sun  might  be  cool  and 
habitable.  Halley1  thought  that  the  appearances  on  the 
eastern  and  western  edges  of  the  sun  at  a  total  eclipse  might 
reasonably  be  expected  to  be  different,  for  the  reason  that 
"  the  eastern  limb  of  the  moon  had  been  exposed  to  the  sun's 
rays  for  a  fortnight,  and  as  a  consequence  it  would  be  natural 
to  expect  that  the  heated  lunar  atmosphere  might  exert  some 
absorbing  effect  on  the  solar  rays,  while  on  the  contrary  the 
western  edge  of  the  moon  being  in  darkness  and  cold  for  two 
weeks  could  exhibit  no  such  absorbing  action." 

The  interest  aroused  in  total  eclipses  was  now  so  great 
that  astronomers  were  determined  to  take  advantage  of 
every  opportunity,  no  matter  how  short  the  time  of  totality 
nor  how  great  distances  it  was  necessary  to  travel  in  order 
to  view  the  eclipses.  The  eclipse  of  July  28,  1851,  was 
visible  in  Norway  and  Sweden,  and  English  astronomy  was 
well  represented  in  the  persons  of  the  astronomer  royal  Airy, 
Hind,  Dawes,  Carrington,  Stephenson,  Gray,  Lassell  and 
Williams.  Although  Faye  *  still  asserted  with  force  that  the 
prominences  were  merely  optical  illusions  or  "  mirages  pro- 
duced near  the  moon's  surface,"  the  general  consensus  of 
opinion  was  that  the  origin  of  the  red  flames  was  to  be 
sought  in  the  sun.  To  this  fire  of  scarlet  hue  Airy  gave  the 
name  of  sierra. 

Any  lingering  doubts  regarding  the  origin  of  this  sierra 
were  forever  dispelled  by  the  observations  made  at  the 
eclipse  of  July  18,  1860,  visible  in  America,  Spain  and 
Northern  Africa.  The  solution  of  the  problem  was  accom- 
plished by  photography  which  was  applied  for  the  first  time 
at  an  eclipse  with  anything  like  success.  As  the  prominences 

1  Phil.  Trans.,  29,  248,  1715. 

2  Memoirs,  R.  A.  S.,  21,  5,  1853. 


MODERN    ECLIPSES    BEFORE    1878  135 

are  red  m  color  to  the  eye  and  as  the  ordinary  photographic 
plate  is  insensitive  to  red  (plates  are  usually  developed 
under  ruby  light)  grave  doubts  were  felt  whether  photo- 
graphs would  be  able  to  portray  the  red  flames.  The  only 
thing  to  do  under  the  circumstances  was  to  "  try  something 
and  see  what  happens  "  (excellent  advice  for  the  scientist 
usually  credited  to  the  late  Professor  H.  A.  Rowland). 
Photography  had  already  been  applied  at  the  eclipse  of  1851 
when  Busch  obtained  some  feeble  impressions  of  the  eclipsed 
sun  by  the  daguerreotype  process.  Photography  was  even 
attempted  in  1842  using  iodized  paper,  but  with  no  results. 
Warren  de  la  Rue  used  the  heliograph  from  Kew,  enlarging 
the  image  before  it  reached  the  photographic  plate,  while 
Father  Secchi  employed  a  six-inch  refractor  without  enlarge- 
ment. Photographs  of  both  observers  were  successful.  De 
la  Rue  was  near  the  Atlantic  in  Spain  while  Secchi  was  on 
the  Mediterranean  Coast,  six  minutes  of  elapsed  time  being 
necessary  for  the  moon's  shadow  to  travel  from  one  station 
to  the  other.  The  conclusions  from  the  1860  eclipse  were: 
i.  The  prominences  are  rich  in  actinic  power  (now  known 
to  be  due  principally  to  the  H  and  K  light  of  calcium).  2. 
As  the  moon  passed  in  front  of  the  sun  it  progressively  cov- 
ered and  uncovered  the  prominences,  thereby  demonstrating 
completely  that  their  origin  is  strictly  solar.  3.  During  the 
six  minutes  of  elapsed  time,  changes  were  noted  in  some 
prominences  but  no  variations  in  others.  4.  The  material 
from  which  the  red  flames  arise  is  found  around  the  whole 
solar  globe.  This  is  the  sierra  of  Airy,  but  later,  called  the 
chromosphere  by  Lockyer. 

The  success  attending  the  eclipse  of  1860  came  almost  at 
the  same  time  with  the  unraveling  of  the  enigma  of  the 
spectrum.  After  two  centuries  of  slow  and  painstaking 
progress,  the  crucial  experiments  had  been  performed  by 
Kirchhoff  by  means  of  which  the  action  of  the  spectro- 
scope was  at  last  understood.  As  a  result,  interest  in  eclipse 
observations  was  no  longer  confined  to  the  astronomer  alone, 
for  many  investigations  were  to  be  undertaken  which  were 
not  confined  by  the  determination  of  exact  time  or  position. 
The  birth  of  physical  astronomy,  or  astrophysics,  insured 


136  ECLIPSES    OF    THE    SUN 

that  henceforth  eclipse  observations  would  be  of  quite  as 
much  interest  to  the  physicist  as  to  the  astronomer,  and  for 
the  interpretation  of  these  observations,  research  work  in  the 
laboratory  was  quite  as  important  as  that  in  the  observa- 
tory. What  spectrum  would  the  prominences  give?  The 
answer  was  not  difficult.  Apparently  the  prominences  were 
not,  as  had  previously  been  thought,  masses  of  photo- 
spheric  material  shot  up  to  great  heights  by  some  explo- 
sive or  volcanic  action  on  the  sun,  since  it  was  evident 
that  the  boomerang-shaped  protuberance  seen  at  the  eclipse 
of  1851  could  hardly  have  existed  under  the  laws  of  gravita- 
tion. It  was  top-heavy  and  could  not  have  hung  there 
above  the  sun  even  for  the  short  space  of  time  it  was  visible 
if  it  had  been  composed  of  the  general  material  forming  the 
sun's  photosphere.  And  then  there  was  the  distinct  differ- 
ence in  color  noted  between  the  red  flames  and  the  body  of 
the  sun.  Manifestly,  since  all  of  the  gases  forming  the  sun 
did  not  take  part  in  the  solar  outburst,  the  prominences 
probably  consisted  of  a  few  gases  only,  perhaps  one  of  their 
chief  constituents  was  the  lightest  of  known  gases,  hy- 
drogen, whose  visible  spectrum  known  from  stellar  investiga- 
tions consisted  of  a  strong  line  in  the  red,  another  in  the  blue, 
and  a  series  of  others  coming  closer  together  as  the  violet 
end  of  the  spectrum  was  approached.  The  red  line  of  hy- 
drogen seemed  to  give  a  color  not  differing  materially  from 
the  red  of  the  prominences.  If  therefore  the  prominences 
were  actually  outbursts  of  hydrogen  gas  heated  to  great 
temperatures  in  the  solar  furnace,  the  eclipse  spectrum 
would  be  vastly  different  from  the  ordinary  solar  spectrum. 
The  band  of  light  would  not  be  continuous  from  the  red  to 
the  violet  end,  nor  would  any  dark  Fraunhofer  lines  be 
visible.  Since  the  prominences  were  probably  gaseous,  their 
spectrum,  as  known  from  the  laws  of  Kirchhoff,  must  consist 
of  bright  lines  on  a  dark  background,  an  emission  spectrum. 
It  appeared  therefore  that  the  prominence  spectrum  would 
consist  of  a  few  bright  lines  only,  the  red  and  blue  lines  of 
hydrogen,  and  the  series  towards  the  more  refrangible  end. 
more  difficult  to  see  on  account  of  the  fact  that  the  human 
eye  is  not  sensitive  to  violet  light. 


OUTER  LAYERS  OF  THE  CHROMOSPHERE 

Photographed  by  Deslandres  by  spectroheliograph  at  Meudon,  France,  on 
June  7,  1919.  Above:  K/3  of  calcium.  Below:  Ha  of  hydrogen.  Note  the 
differences  in  appearance  of  the  two  photographs  and  the  fine  detail  that 


TEST  OF  RADIAL  MOTIONS  IN  THE  SUN 

Photographed  by  Deslandres  at  Meudon  with  the  Ki  ray  of  calcium, 

April  ii,  1910. 


MODERN    ECLIPSES    BEFORE    1878  137 

But  there  was  no  eclipse  on  which  to  "  try  and  see  what 
happened  "  until  August  18,  1868,  so  it  was  necessary  for 
the  physicists  and  astronomers  to  possess  their  souls  with 
patience;  but  alas!  the  eclipse  was  visible  only  in  far-off 
India,  the  Malay  peninsula  and  Siam.  The  distances  were 
great,  but  the  problems  were  important,  and  accordingly 
several  expeditions,  two  British,  two  French,  one  German 
and  one  Spanish  were  found  in  the  eclipse  track.  The 
greatest  success  attended  the  observation  of  Janssen.  The 
slit  of  his  spectroscope  directed  to  the  edge  of  the  sun  re- 
vealed the  spectrum  of  the  prominences  consisting,  as  had 
been  thought,  of  a  series  of  bright  emission  lines,  most 
prominent  among  which  were  three  lines,  the  red  and  the 
blue  lines  clearly  belonging  to  hydrogen,  but  one  of  almost 
equal  brilliancy  in  the  yellow.  The  color  of  the  yellow  line 
seemed  to  match  the  D-lines  of  sodium,  ever-present  in 
laboratory  experiments,  but  why  should  the  gas  sodium, 
comparatively  heavy,  be  found  in  the  prominences? 

The  brilliancy  of  the  prominence  lines  was  so  remark- 
able that  Janssen  determined  to  seek  them  again  after  the 
eclipse  was  over.  If  they  were  solar  in  origin  they  must 
be  found  on  the  sun  every  day,  varying  it  is  true  in  shape 
and  dimensions.  The  only  reason  why  the  prominences 
cannot  be  seen  any  day  is  the  same  reason  why  the  stars 
are  not  visible  in  daylight,  —  the  glare  of  the  earth's  at- 
mosphere, especially  when  close  to  the  sun,  being  very  great. 
Abolish  the  atmosphere  where  the  observer  worked,  and  the 
stars  and  prominences  would  at  once  become  visible;  and 
since  the  moon  has  little  or  no  appreciable  atmosphere,  the 
prominences  would  be  seen  each  day  without  an  eclipse  to 
the  "  man  in  the  moon  "  if  such  a  person  existed.  But  how 
get  rid  of,  or  diminish,  the  glare  of  the  earth's  atmosphere 
to  such  an  extent  that  the  prominence  luminosity  will  be 
more  intensive  than  the  light  of  the  earth's  atmosphere? 
This  feat  can  be  accomplished  by  the  spectroscope.  The 
emission  lines,  C  and  F  of  hydrogen,  found  in  prominences, 
are  monochromatic  in  character,  that  is,  these  lines  betake 
approximately  the  nature  of  mathematical  lines  and  show  no 
appreciable  width.  As  a  matter  of  fact,  it  is  impossible  to 


138  ECLIPSES    OF    THE    SUN 

diminish  their  intensity  by  increasing  the  dispersion  of  the 
spectroscope,  whether  this  increase  is  accomplished  by  the 
addition  of  extra  prisms  or  by  the  employment  of  a  grating. 
Increased  dispersion,  however,  spreads  out  these  lines  in 
the  spectrum  to  greater  distances  apart,  but  their  inten- 
sities are  not  thereby  diminished.  On  the  other  hand,  the 
continuous  background  of  the  solar  spectrum  can  be  weak- 
ened at  will  by  merely  increasing  the  amount  of  the  disper- 
sion, a  fact  which  is  at  once  evident  since  the  light 
passing  through  the  slit  is  by  an  increase  of  prisms  spread 
over  a  greater  area.  Reflected  sunlight,  whether  from 
the  moon's  surface,  from  the  planets,  from  a  silvered 
mirror  or  from  particles  of  dust  in  the  earth's  atmos- 
phere, gives  the  solar  spectrum.  Consequently,  the  promi- 
nences may  be  made  visible  by  the  spectroscope  by  the 
simple  process  of  increasing  the  dispersion  to  such  an 
extent  that  they  can  shine  by  contrast  with  the  weakened 
atmospheric  glare.  As  a  result  of  these  ideas,  Janssen 
looked  for  and  found  the  prominences  after  the  eclipse  was 
a  thing  of  the  past.  The  same  or  similar  ideas  had  oc- 
curred to  other  workers  with  the  spectroscope,  notably  to 
Huggins  and  to  Lockyer  in  England.  Without  having  been 
present  at  the  eclipse,  the  latter  tried  for  the  prominences 
and  found  them  for  the  first  time  on  October  20,  1868. 
Lockyer  sent  a  record  of  his  observations  to  the  French 
Academy,  and  without  having  heard  of  the  Englishman's 
results,  Janssen  sent  on  to  Paris  the  report  of  the  work  he 
had  done  at  the  eclipse  and  afterwards.  By  a  strange  coin- 
cidence, the  papers  from  both  investigators  were  read  at  the 
same  sitting  of  the  Academy,  in  honor  of  which  event  a 
medal  was  struck  bearing  the  likeness  of  both  Lockyer  and 
Janssen. 

Thus  in  the  moment  when  the  chemical  nature  of  the 
prominences  was  discovered  by  the  spectroscope,  these  ob- 
jects ceased  to  be  phenomena  confined  to  eclipses  only.  As 
a  happy  and  most  fortunate  result,  a  two-fold  benefit  thus 
accrued  to  the  astronomer:  freed  from  the  necessity  of  ob- 
serving prominences  during  the  all  too  brief  moments  of  a 
total  eclipse  he  could  devote  his  energies  to  other  investiga- 


MODERN    ECLIPSES    BEFORE    1878  139 

tions.  Not  only  could  prominences  be  observed  without  an 
eclipse ,  but  by  the  same  methods,  researches  could  be  carried 
out  on  the  solar  envelope  from  which  prominences  arose,  the 
chromosphere.  Frequent  violent  eruptions  on  the  sun 
carried  the  solar  flames  up  to  great  distances  and  with  enor- 
mous velocities,  and  these  phenomena  were  observed  visually 
by  Young,  Lockyer,  Tacchini  and  a  host  of  other  investiga- 
tors, many  important  researches  1  being  carried  out.  The  in- 
vention of  the  spectro-heliograph  in  1893,  independently  and 
almost  simultaneously,  by  Hale  and  Deslandres  permitted 
an  attack  on  these  problems  by  the  help  of  photography. 

The  great  triumph  of  the  spectroscope  in  1868  gave  evi- 
dence to  the  solar  astronomer  of  the  important  problems 
awaiting  solution,  and  it  appeared  almost  certain  that  each 
observation  carried  out  with  care  would  be  a  valuable  dis- 
covery. What  was  the  corona?  Did  it  shine  by  its  own 
light,  or  by  reflected  sun's  light?  Was  there  any  connec- 
tion between  the  prominences  and  the  corona?  How  far 
out  did  the  corona  extend,  and  were  there  any  changes  in 
its  form  that  could  be  detected?  Was  the  explanation  of 
the  dark  Fraunhofer  lines  of  the  solar  spectrum  the  true 
one,  and  was  it  possible  that  eclipses  could  help  in  the 
problem? 

The  eclipse  of  August  7,  1869,  crossed  America  diag- 
onally from  Alaska  to  North  Carolina.  The  United 
States  government  made  a  large  appropriation  to  help  de- 
fray the  expenses,  and  as  a  result  of  this  and  also  on  ac- 
count of  the  favorable  time  of  year,  the  eclipse  track  in 
the  United  States  was  almost  one  continuous  observatory, 
so  thickly  were  the  astronomers  scattered.  To  give  a  list 
of  those  who  saw  the  eclipse  would  be  practically  equiva- 
lent to  making  a  record  of  every  astronomer  of  any  impor- 
tance who  was  then  living  in  the  United  States.  Those  too 
"  were  the  good  old  days  "  before  railroad  executives  were 
harassed  by  government  regulations  and  Interstate  Com- 
merce Commissions  and  when  free  transportation  for  pas- 
sengers and  goods  could  be  furnished.  The  following  is 
copied  from  the  Report  on  Observations  of  the  Total 

i  See  Young,  The  Sun. 


140  ECLIPSES    OF    THE    SUN 

Eclipse  of  the  Sun,  August  7,  1869.  On  page  3,  J.  H.  C. 
Coffin,  U.  S.  N.,  Superintendent  of  the  Nautical  Almanac, 
reports,  "  Col.  Thomas  A.  Scott,  vice-president  of  the  Penn- 
sylvania Central  Railroad,  furnished  a  special  car,  and  with 
the  cordial  cooperation  of  Mr.  Robert  Harris,  general  super- 
intendent of  the  Chicago,  Burlington  and  Quincy,  and  Mr. 
C.  E.  Perkins  of  the  Burlington  and  Missouri  Railroad, 
provided  free  transportation  for  these  parties,  with  all  their 
instruments  and  apparatus  to  and  from  the  places  of  ob- 
servation. Free  passes  to  ten  or  twelve  others  over  the 
same  routes  were  also  granted/'  In  the  same  report,  page 
115,  Professor  Morton  estimates  that  "  an  expense  of 
$1500  was  spared  the  government  appropriation." 

Fortunately,  clear  skies  greeted  the  observing  parties. 
Little  of  the  important  work  accomplished  will  be  noted 
in  detail  here.  Spectroscopically,  the  most  valuable  dis- 
covery was  that  the  spectrum  of  the  corona  was  continuous 
but  was  traversed  by  a  single  green  ray.  This  green  line 
was  detected  independently  by  both  Harkness  and  Young, 
the  latter  identifying  its  position  as  coinciding  with  the  line 
numbered  1474  on  Kirchhoffs  scale.  But  since  this  line 
1474  is  due  to  iron,  it  was  surprising  and  perplexing  in  the 
highest  degree  to  find  it  present  in  the  corona  and  reaching 
such  great  heights  above  the  sun's  surface.  In  spite  of  the 
apparent  coincidence,  it  was  evident  that  the  substance 
causing  the  green  line  was  not  iron.  To  it  the  name 
coronium  was  given,  —  and  today  after  more  than  a  half 
century  of  active  research  we  know  little  more  of  coronium 
than  when  it  was  first  discovered.  At  this  same  eclipse, 
Professor  E.  C.  Pickering  employed  a  portrait  lens,  thus 
recognizing  its  value  in  the  portrayal  of  the  heavens.  As 
he  was  located  at  Mt.  Pleasant,  a  little  farther  west  than  the 
rest  of  the  expeditions,  to  him  belongs  the  honor  of  securing 
the  first  successful  photograph  of  the  corona  in  America. 
Other  photographs  were  secured  by  Winlock  and  others  on  a 
larger  scale  and  with  good  definition  which  showed,  the 
corona  and  prominences. 

The  United  States  Congress  appropriated  the  sum  of 
$29,000  for  observations  at  the  eclipse  of  December  22, 


MODERN    ECLIPSES    BEFORE    1878  141 

1870,  visible  in  Spain,  Northern  Africa,  Sicily,  Greece  and 
Turkey.  America  was  represented  in  the  eclipse  track  by 
the  following:  Pierce,  Newcomb,  Harkness,  Hall,  East- 
man, Winlock,  Young,  Langley,  Pickering,  Peters,  Watson, 
Clark,  Ernst,  Willard,  Ross,  Gannet,  and  General  Abbott. 
The  British  government  granted  £2000  and  a  ship.  A 
great  variety  of  observations  were  projected  and  only  par- 
tially carried  out  on  account  of  clouds  that  prevailed  almost 
everywhere,  observations  photographic,  spectroscopic, 
photometric  and  polariscopic.  Lockyer  met  shipwreck  and 
clouds,  but  was  rewarded  in  the  end  by  a  glimpse  of  the 
corona  for  one  brief  second  and  a  half!  Janssen  made 
good  his  exit  from  beleaguered  Paris  in  a  balloon,  but  though 
he  succeeded  in  escaping  from  the  bullets  of  the  Germans, 
he  was  forced  to  capitulate  to  obscuration  by  the  clouds. 

The  most  conspicuous  success  awaited  the  efforts  of  Pro- 
fessor C.  A.  Young  of  Princeton  who  had  foretold,  and 
whose  eye  was  the  first  to  see  the  "  flash  spectrum."  Ac- 
cording to  the  theory  of  Kirchhoff  (p.  97)  the  spectrum 
of  the  photosphere  would  be  continuous  from  red  to  violet 
without  any  dark  lines  were  it  not  for  the  overlying  solar 
atmosphere.  Here  in  the  so-called  reversing  layer,  the 
gases  are  at  a  lower  temperature  than  in  the  photo- 
sphere, and  on  account  of  these  cooler  conditions  the 
photospheric  light  is  absorbed  by  the  reversing  layer, 
and  the  resultant  spectrum  is  that  of  dark  lines  on 
a  bright  background.  As  already  explained,  if  the  photo- 
sphere could  be  removed,  then  the  gases  forming  the  revers- 
ing layer,  since  they  are  at  a  high  temperature,  would  give 
a  series  of  bright  lines  on  a  dark  background,  which  is 
technically  called  a  "  reversal  "  of  the  Fraunhofer  spec- 
trum. To  describe  the  appearance  in  1870,  one  cannot  do 
better  than  to  quote  from  the  words  of  the  discoverer:1 
"  The  observation  is  possible  only  under  peculiar  circum- 
stances. At  a  total  eclipse  of  the  sun,  at  the  moment  when 
the  advancing  moon  has  just  covered  the  sun's  disk,  the  solar 
atmosphere  of  course  projects  somewhat  at  the  point  where 
the  last  ray  of  sunlight  has  disappeared.  If  the  spectro- 

1  Young,   The  Sun,  p.   83. 


142  ECLIPSES    OF    THE    SUN 

scope  be  then  adjusted  with  its  slit  tangent  to  the  sun's 
image  at  the  point  of  contact,  a  most  beautiful  phenomenon 
is  seen.  As  the  moon  advances,  making  narrower  and  nar- 
rower the  remaining  sickle  of  the  solar  disk,  the  dark  lines 
of  the  spectrum  for  the  most  part  remain  sensibly  un- 
changed, though  becoming  somewhat  more  intense.  A  few, 
however,  begin  to  fade  out,  and  some  even  turn  palely 
bright  a  minute  or  two  before  totality  begins.  But  the 
moment  the  sun  is  hidden,  through  the  whole  length  of  the 
spectrum,  in  the  red,  the  green,  the  violet,  the  bright  lines 
flash  out  by  hundreds  and  thousands,  almost  startlingly;  as 
suddenly  as  stars  from  a  bursting  rockethead,  and  as 
evanescent,  for  the  whole  thing  is  over  in  two  or  three 
seconds.  The  layer  seems  to  be  only  something  under  a 
thousand  miles  in  thickness,  and  the  moon's  motion  covers 
it  very  quickly. 

"  The  phenomenon,  though  looked  for  at  the  first  eclipses 
after  solar  spectroscopy  began  to  be  a  science,  was  missed 
in  1868  and  1869,  as  the  requisite  adjustments  are  delicate, 
and  was  first  actually  observed  only  in  1870." 

The  same  phenomenon  was  witnessed  by  Pye,  a  member 
of  Young's  party.  The  bright  lines  were  so  numerous  that 
the  impression  was  gained  that  every  one  of  the  thousands 
of  Fraunhofer  lines  was  reversed  from  dark  to  bright,  wrhile 
"  the  phenomenon  was  so  sudden,  so  unexpected,  and  so 
wonderfully  beautiful  as  to  force  an  involuntary  exclama- 
tion." 1  Professor  Young  called  this  sudden  transition  the 
flash  spectrum.  The  same  phenomenon  was  witnessed  at 
the  eclipse  of  December  12  of  the  following  year  by 
Maclear,  Herschel,  Lockyer  and  Fyers,  the  eclipse  being 
total  in  India,  Ceylon  and  Northern  Australia  (where  clouds 
interfered).  Again  at  the  annular  eclipse  of  June  6,  1872, 
seen  by  Pogson  in  India,  and  at  the  total  eclipse  of  April 
1 6,  1874,  witnessed  by  Stone  in  South  Africa,  the  flash 
spectrum  was  observed.  This  is  one  of  the  most  interest- 
ingly beautiful  and  important  of  the  phenomena  connected 
with  total  eclipses.  During  the  past  half  century  the  flash 
spectrum  has  been  carefully  observed  at  each  eclipse.  More 

1  Memoirs,  R.  A.  S.,  41,  435. 


MODERN    ECLIPSES    BEFORE    1878  143 

recently  it  has  been  photographed  in  exquisite  detail,  and 
has  also  been  photographed  at  Mt.  Wilson  without  an 
eclipse.  (See  Chapter  XV.) 

Apparently,  it  had  been  almost  definitely  settled  that 
the  corona  is  a  truly  solar  appendage,  for  in  addition 
to  the  spectroscopic  evidence  in  the  matter,  the  photo- 
graphs  in  1871  showed  the  same  details  in  the  coronal 
streamers  from  observing  stations  widely  separated,  an 
effect  which  could  not  possibly  take  place  if  the  corona 
was  terrestrial  in  origin.  Such  then  was  the  state  of  scien- 
tific information  regarding  the  corona  as  it  existed  before 
the  eclipse  of  1878  which  was  visible  in  America.  Little  was 
known  with  exactitude,  there  was  very  much  of  perplexity 
and  uncertainty  and  doubt.  The  unsatisfactory  nature  of 
the  whole  subject  may  perhaps  be  envisaged  in  one  sentence 
copied  from  a  book  by  one  of  the  most  assiduous  of  spectro- 
scopic investigators  of  the  time,  Norman  Lockyer.  "  Now 
the  whole  phenomenon  of  the  corona  may  be  defined  in  two 
words,  cool  prominences" 

If  the  knowledge  of  the  corona  was  disappointing  and 
perplexing,  the  decisions  of  the  spectroscope  regarding  the 
reversing  layer  were  equally  uncertain.  Kirchhoffs  theory 
demanded  that  there  should  be  a  layer  of  vapors  relatively 
cool  surrounding  the  photosphere.  Where  was  this  layer, 
close  to  or  far  away  from  the  sun?  Was  the  layer  thin,  or 
one  more  extensive?  At  the  eclipse  of  1870,  Young  had 
discovered  a  flash  spectrum  apparently  of  thousands  of 
lines  suddenly  turned  from  dark  to  bright.  This  phe- 
nomenon was  visible  for  only  two  or  three  seconds  at  the 
beginning  and  ending  of  totality,  and  in  consequence,  the 
reversing  layer  must  be  very  shallow,  approximately  600 
miles  in  thickness.  But  were  all  of  the  Fraunhofer  lines 
turned  from  dark  to  bright?  Since  the  appearance  was 
visible  during  the  few  hurried  and  excited  seconds  of  an 
eclipse,  it  was  manifest  that  no  exact  comparison  with 
Fraunhofer  lines  was  possible,  nor  would  be  possible  until 
the  time  should  arrive  when  the  flash  spectrum  could  be  re- 
corded by  photography.  Moreover  some  spectral  lines  were 
seen  to  turn  bright  half  a  minute  or  more  before  the  be- 


144  ECLIPSES    OF    THE    SUN 

ginning  of  totality,  so  that  it  was  evident  that  not  all  of  the 
reversing  layer  was  confined  to  the  6oo-mile  limit.  Still 
other  difficulties  presented  themselves.  If  the  absorption 
did  take  place  in  a  layer  of  the  solar  atmosphere,  then  the 
light  of  the  photosphere  passing  tangentially  through  this 
layer  at  the  sun's  limb  should  experience  an  absorption 
greater  in  amount  than  that  from  the  sun's  center.  In  conse- 
quence, the  spectra  of  the  center  and  limb  of  the  sun  were 
compared  in  great  detail,  with  the  resulting  discovery  that 
any  differences  in  the  relative  intensities  of  the  lines  were 
of  such  minor  character  that  they  could  readily  be  explained 
by  the  slight  darkening  of  the  limb  compared  with 
that  of  the  center.  Kirchhoff  believed  that  the  spectral 
lines  of  any  element  like  sodium  were  characteristic, 
or  in  other  words,  the  same  element  gave  always  the 
same  series  of  lines  no  matter  how  the  element  was 
vaporized.  But  when  the  flame  spectrum  was  com- 
pared with  that  of  the  higher  temperature  of  the  elec- 
tric arc,  and  the  still  greater  temperature  of  the  electric 
spark,  great  differences  in  the  relative  intensities  of  the  lines 
were  at  once  observed.  Lockyer  was  the  first  to  call  atten- 
tion to,  and  emphasize  the  importance  of  these  changes, 
Take  the  element  calcium,  for  instance.  With  a  Bunsen 
flame  and  with  small  dispersion,  the  chief  line  visible  is  in 
the  red  end  of  the  spectrum.  At  the  temperature  of  the 
arc,  the  strongest  line  is  in  the  blue,  at  42  2 7 A.  The  red 
line  is  also  visible  and  also  two  lines  in  the  violet,  the  H  and 
K  lines  of  the  solar  spectrum.  Increase  the  temperature 
of  excitation  by  using  the  electric  spark,  and  the  two  violet 
lines  greatly  increase  their  intensities  and  become  much 
stronger  than  the  blue  line,  while  the  red  line  practically 
disappears.  Similar  changes  of  intensity  were  found  by 
Lockyer  in  magnesium,  lithium,  iron  and  other  elements 
examined,  and  these  conclusions  have  been  abundantly 
verified  by  all  observers  since  his  time.  Still  further  diffi- 
culties presented  themselves.  When  the  wave-lengths  for 
the  various  metals  were  determined,  it  was  discovered  that 
different  elements  had  spectral  lines  with  apparently  iden- 
tical wave-lengths.  It  was  found  that  some. of  these  com- 


MODERN    ECLIPSES    BEFORE    1878  145 

mon  lines  were  due  to  impurities  in  the  metals  examined, 
but  when  these  lines  were  omitted  from  consideration  there 
were  still  many  lines  evidently  common  to  two  or  more 
elements.  It  was  consequently  manifest  that  the  identifica- 
tion of  lines  in  the  solar  spectrum  was  not  the  simple  opera- 
tion Kirchhoff  supposed  it  to  be,  and  that  "  the  more 
observations  were  accumulated  the  more  the  spectroscopic 
difficulties  increased.7'  l  Although  some  of  the  theories  and 
conclusions  of  Lockyer  were  incorrect,  nevertheless  spec- 
troscopists  are  under  a  great  debt  to  him  for  the  splendid 
series  of  researches  carried  out  both  in  the  laboratory  and 
in  the  observatory. 

To  solve  some  of  these  difficulties,  Lockyer  busily  in- 
vestigated the  spectra  of  many  elements  by  his  well-known 
method  of  long  and  short  lines.  If  an  electric  arc  is  ar- 
ranged horizontally  and  its  image  is  projected  on  the 
vertical  slit  of  the  spectroscope,  it  is  seen  at  once  that  the 
lengths  of  the  lines  in  the  spectrum  vary  considerably. 
Since  the  core  of  the  vapor  between  the  two  carbon  poles 
must  be  much  hotter  than  that  on  the  outside  edges,  it  was 
evident  to  Lockyer  that  the  short  lines  were  high  tempera- 
ture lines  that  become  visible  only  at  the  hottest  point,  that 
of  the  core,  while  the  long  lines  were  those  which  could 
exist  at  different  temperatures,  at  the  great  heat  of  the 
center  of  the  arc  and  at  the  lesser  temperatures  of  the  out- 
side edges.  This  method  of  long  and  short  lines  thus  ap- 
peared to  give  a  ready  and  convenient  means  of  separating 
the  lines  of  highest  temperature,  which  were  comparatively 
few  in  number,  from  the  balance  of  the  spectrum. 

By  arranging  the  spectra  of  stars  in  an  orderly  series,  be- 
ginning with  the  white  stars,  and  passing  through  yellow 
stars  to  those  red  in  color,  it  was  noted  by  Lockyer  that 
there  was  a  steady  increase  in  the  total  number  of  lines  in 
the  spectra,  there  being  few  lines  in  the  white  stars  other 
than  the  hydrogen  series,  while  the  red  stars  possess  an 
enormous  number  of  spectral  lines.  Thus  in  the  progres- 
sion in  the  reverse  direction,  from  red  stars  back  to  white, 
the  spectra  show  fewer  and  fewer  lines  and  become  simpler 

1  Lockyer,  The  Chemistry  of  the  Sun,  p.  176. 


146  ECLIPSES    OF    THE    SUN 

in  character.  The  same  progression  from  red  to  white  stars 
saw  an  increase  in  intensity  in  the  H  and  K  lines  due  to 
calcium.  And  as  Lockyer  was  the  first  to  recognize  the  im- 
portance of  temperature  in  altering  the  character  of  spectra, 
so  to  him  likewise  belongs  the  great  honor  of  recognizing 
that  changes  in  stellar  spectra  show  that  the  white  stars  are 
hotter  than  the  yellow  and  these  in  turn  are  hotter  than  the 
red.  (Present-day  researches  show  that  as  the  stars  increase 
in  temperature  in  the  stellar  series,  the  H  and  K  lines  do 
not  steadily  increase  in  intensity  but  reach  a  maximum 
strength,  then  begin  to  fade  away  and  are  entirely  wanting 
in  the  hottest  stars,  the  blue-white  helium  stars  of  spectral 
type  B  and  stars  of  type  O.) 

Momentous  in  the  highest  degree  therefore  were  the  dis- 
coveries of  Lockyer  showing  the  importance  of  temperature 
in  the  interpretation  of  spectra,  —  but  his  explanations 
of  the  underlying  causes  have  not  borne  the  weight  of  time. 
His  was  the  dissociation  theory  that  the  chemical  atoms 
were  continually  broken  up  into  elements  less  complex  in 
structure,  which  exhibited  simpler  and  simpler  forms  of 
spectra  as  higher  and  still  higher  temperatures  were  reached. 
As  the  white  stars  are  those  of  the  highest  temperature,  and 
as  they  show  practically  nothing  but  the  spectrum  of  hy- 
drogen which  is  the  element  of  smallest  atomic  weight,  it 
must  represent  the  simplest  element  with  the  simplest  spec- 
trum. (The  most  recent  researches  show  that  though  there 
may  be  dissociation  of  the  chemical  atoms  this  takes  place  in 
a  manner  vastly  different  from  the  dissociation  demanded  by 
Lockyer's  theory). 

The  sun  being  but  a  typical  yellow  star,  this  theory  can 
be  put  to  the  test  in  attempting  to  explain  the  phenomena 
of  the  solar  atmosphere  as  revealed  by  eclipses.  Lockyer 
assumed  "  that  in  the  reversing  layers  of  the  sun  and 
stars  various  degrees  of  chemical  dissociation  are  at 
work,  which  dissociation  prevents  the  coming  together 
of  the  atoms  which,  at  the  temperature  of  the  earth 
and  at  all  artificial  temperatures  attained  here,  compose  the 
metals,  the  metalloids  and  the  compounds."  x  In  conse- 

1  The  Chemistry  of  the  Sun,  201. 


MODERN    ECLIPSES    BEFORE    1878  14/1 

quence,  there  were  grave  doubts  expressed  by  Lockye 
whether  the  chemical  elements  known  from  laboratory  ex 
periments  could  at  all  exist  at  the  great  heat  of  the  sur 
except  in  the  cooler  parts  of  its  atmosphere.  It  wa 
imagined  by  him  that  this  atmosphere  consisted  of  succes 
sive  layers  "  like  the  skins  of  an  onion/'  the  layers  next  the 
sun  obviously  being  the  hottest.  Hence  in  the  interior  layer! 
could  exist  only  "  those  constituents  of  the  elementary 
bodies  which  can  resist  the  greater  heat  of  these  regions/ 
The  spectrum  of  these  inside  layers,  if  such  a  spectrun 
could  be  obtained  apart  from  that  of  the  rest  of  th( 
sun,  would  therefore  not  be  a  reversal  of  the  Fraunhofei 
spectrum.  According  to  Lockyer's  hypothesis,  the  whoh 
of  the  solar  atmosphere  is  effective  in  the  productior 
of  absorption.  Young's  observation  of  the  flash  spec 
trum  demanded  a  shallow  reversing  layer  of  a  few  hun 
dred  miles,  but  Lockyer's  theory  refused  to  admit  th< 
existence  of  a  reversing  layer  separate  from  the  super 
incumbent  strata.  In  other  words,  there  could  be  IK 
division  possible  into  reversing  layer,  chromosphere  anc 
corona;  these  were  but  different  manifestations  of  the  solai 
atmosphere,  the  corona  being  regarded  as  the  outermost  am 
cooler  parts  of  an  atmosphere  having  a  composite  existena 
and  obeying  the  laws  of  gravitation.  In  consequence  of  this 
theory,  although  Lockyer  had  himself  witnessed  the  flast 
spectrum  at  the  eclipse  of  1871,  he  at  first  was  forcec 
to  doubt,  and  then  actually  to  deny  the  existence  oJ 
the  shallow  reversing  layer.  The  flashing  out  of  lines 
that  he  observed  "  has  been  called  the  reversing  layer; 
but  I  do  not  now  (1881)  believe  that  it  is  the  reversing 
layer  for  a  moment,  for,  when  it  comes  to  be  examined 
we  shall  probably  find  that  scarcely  any  of  the  Fraun- 
hofer  lines  owe  their  origin  to  it,  and  we  shall  have  % 
spectrum  which  is  not  a  counterpart  of  the  solar  spec- 
trum." (Loc.  cit.^p.  360.)  The  solution  of  the  problen 
could  not  be  effected  visually  during  the  brief  seconds  avail- 
able at  a  total  eclipse  for  observation  of  the  flash  spec- 
trum. The  interpretation  by  one  astronomer  of  what  was 
observed  should  be  entitled  to  as  much  weight  as  the  opin- 


148  ECLIPSES    OF    THE    SUN 

ion  of  another.  There  was  no  hope  of  a  solution  of  the 
problem  until  the  time  should  arrive  when  photography 
could  come  to  the  rescue  by  furnishing  a  permanent  record 
of  the  flash  spectrum  which  could  be  compared  line  by  line 
with  the  Fraunhofer  spectrum  in  order  to  see  whether  the 
one  spectrum  is  the  exact  reversal  of  the  other. 

While  these  observations,  epoch-making  in  their  impor- 
tance, were  being  made  on  prominences  and  reversing  layer, 
the  corona  was  not  forgotten.  Strange  as  it  may  now  seem, 
there  were  still  many  astronomers  of  repute  who  believed 
that  the  origin  of  the  coronal  light  should  be  sought,  not  in 
solar  but  in  lunar  or  terrestrial  causes.  There  were  even  two 
theories  based  on  the  moon,  one  that  the  corona  was  due  to 
the  diffraction  of  solar  rays  which  pass  near  the  moon's  edge; 
the  other,  that  the  phenomenon  was  due  to  reflection  of 
solar  rays  from  the  irregularities  of  the  moon's  surface. 
Another  curious  theory  which  found  great  favor  at  the  time 
was  that  the  corona  was  due  simply  to  glare  in  the  earth's 
atmosphere.  As  a  result  of  this  hypothesis  the  corona  would 
necessarily  be  a  phenomenon  entirely  local  in  its  structural 
character,  details  appearing  differently  to  observers  at  sepa- 
rate localities.  If  due  simply  to  atmospheric  glare,  the 
coronal  details  should  be  found  projected  also  on  the  dark 
moon. 

There  were  now  available  four  different  methods  for 
attacking  the  corona:  first,  visual  observations  by  the  naked 
eye,  supplemented  by  the  telescope;  second,  photography; 
third,  polariscopic  observations;  and  fourth,  the  spectro- 
scope. The  polariscope  had  already  shown  that  part,  at 
least,  of  the  coronal  light  was  reflected  sunlight.  This  con- 
clusion was  corroborated  by  the  discovery  by  Janssen  at  the 
eclipse  of  1871  of  dark  Fraunhofer  lines  in  the  coronal 
spectrum,  chief  among  which  the  D  line  of  sodium  was 
recognized.  The  green  emission  line  discovered  at  the 
eclipse  of  1869  was  observed  again  in  1870  and  1871, 
Tennant  in  the  latter  year  discovering  that  this  ray  was 
quite  as  conspicuous  in  a  rift  in  the  coronal  light  as  in 
the  adjacent  streamers.  The  corona  appeared  thus  to 
be  shining  from  the  luminosity  of  some  unknown  gas,  which 


1893 
Chile 


igoo 
Georgia 
U.S.A. 


I8Q5 

India 


1901 
Sumatra 


THE  CORONA  PHOTOGRAPHED  BY  LICK  OBSERVATORY  PARTIES  IN 
DIFFERENT  QUARTERS  OF  THE  WORLD 


PROMINENCES  SEEN  IN  PROJECTION  AGAINST  THE  SUN'S  DISK 
Photograph  from  Mt.  Wilson  Observatory. 


MODERN    ECLIPSES    BEFORE    1878  149 

strange  to  say  shone  as  strongly  in  the  dark  regions  of  the 
corona  as  in  the  bright  streamers. 

In  1871  for  the  first  time,  and  due  to  a  suggestion  by 
Young,  a  slitless  spectroscope  was  tried.  With  the  use  of 
such  an  instrument  at  mid-totality,  the  emission  lines  ap- 
peared as  rings  of  light,  from  the  extent  of  which  one  could 
ascertain  the  height  of  the  various  gases  forming  the  corona. 
By  the  help  of  photography  in  1871,  Lockyer  showed  that 
hydrogen  extended  uniformly  about  the  sun  to  the  enormous 
height  of  200,000  miles.  Could  the  solar  atmosphere  pos- 
sibly extend  to  this  colossal  distance?  Let  us  reason  by 
analogy  with  what  we  know  of  the  earth.  Its  atmosphere,  as 
known  from  observations  of  meteors,  extends  about  one 
hundred  miles  above  sea-level.  The  sun  is  more  than  one 
hundred  times  the  diameter  of  the  earth  and  with  the  same 
relative  distribution  the  solar  atmosphere  might  conceivably 
extend  10,000  miles  from  the  solar  surface.  But  an  atmos- 
phere possesses  mass  and  is  obedient  to  the  law  of  gravita- 
tion, and  since  gravity  on  the  sun  is  twenty-seven  times  that 
on  the  earth,  we  would  expect  that  a  true  atmosphere  on  the 
sun  could  not  extend  to  elevations  of  more  than  4,000  miles. 
But  the  hydrogen  observed  was  at  distances  of  200,000 
miles,  with  the  green  ring  reaching  the  still  greater  extent 
of  300,000  miles,  whereas  the  coronal  streamers  were  seen 
to  stretch  out  several  millions  of  miles  from  the  sun's  surface. 
Apparently,  the  spectroscope  had  not  solved  many  of  the 
difficulties  of  the  coronal  structure,  but  rather  had  succeeded 
only  in  complicating  matters,  for  to  add  to  the  difficulties 
already  great,  it  was  now  necessary  to  explain  how  it  was 
possible  that  the  luminous  gases  hydrogen  and  coronium 
could  extend  to  the  very  great  distances  revealed  by  the  spec- 
troscope. No  terrestrial  origin  could  be  found  for  the  green 
coronal  line,  nor  did  Young's  discovery  in  1876,  that  Kirch- 
hoff's  "  1474  "  was  a  double  line  help  solve  the  problem. 
The  perplexities  were  indeed  very  great.  A  faint  ray  of 
hope  appeared  in  an  unexpected  quarter.  In  1866,  shortly 
after  the  great  November  meteor  shower,  Schiaparelli 
proved  that  the  Perseid  meteors  moved  in  the  same  path  as 
Tuttle's  comet  of  1862,  while  the  Leonids  and  the  Temple 


150  ECLIPSES    OF    THE    SUN 

comet  had  identical  orbits.  This  double  coincidence  between 
meteor  and  cometary  orbits  was  corroborated  in  1872  when 
it  was  found  that  the  Andromedes,  or  Bielids  as  they  are  now 
called,  had  the  same  path  about  the  sun  as  the  lost  Biela 
comet.  The  importance  of  meteors  in  any  cosmical  proc- 
ess was  thus  realized,  and  it  was  but  natural  that  attempts 
should  be  made  to  solve  the  coronal  puzzle  by  means  of  the 
meteoric  hypothesis,  —  but  as  we  shall  see  later,  with  little 
success.  Newton  and  Cleveland  Abbe  in  America  and 
Lockyer  in  England  pinned  most  faith  to  the  meteoric  ex- 
planations. 


CHAPTER  IX 

NINETEENTH    CENTURY    ECLIPSES   AFTER   1878 

total  eclipse  of  July  28,  1878,  was  observed  in 
the  United  States  from  Wyoming  to  Texas.  On 
account  of  the  great  interest  in  and  the  vast  im- 
portance of  the  problems  to  be  solved,  the  eclipse  was  widely 
observed  from  some  twelve  stations  by  about  one  hundred 
astronomers. 

Ever  since  the  discovery  of  the  sun-spot  period  in  1843 
and  the  finding  that  the  earth's  magnetism  possessed  the 
same  period,  astronomers  had  been  on  the  alert  to  ascertain 
whether  other  solar  phenomena  moved  in  cycles  parallel  to 
the  sun-spot  curve.  It  had  already  been  found  that  both 
prominences  and  faculae  were  more  numerous  when  spots 
were  great  in  number,  and  naturally  the  question  was  asked 
whether  the  coronal  streamers  did  not  originate  with  greater 
energy  when  spots  were  at  a  maximum.  Here  was  an  oppor- 
tunity to  test  any  conclusions,  for  Wolf's  sun-spot  number 
for  July  1878  was  o.i,  representing  a  minimum  of  spots, 
while  the  number  for  December  1870  was  135.4,  a  time  of 
maximum  of  spots. 

In  the  state  of  Colorado  the  moon's  shadow  path  crossed 
the  Rocky  Mountains.  Pike's  Peak  (altitude  14,400  feet) 
was  occupied  by  Professor  Langley  and  party.  At  their 
station  before  the  day  of  the  eclipse  there  was  hail,  rain, 
sleet,  snow,  fog  and  every  form  of  bad  weather  which  con- 
tinued for  a  week,  and  to  add  to  the  discomforts,  the  horrors 
of  mountain  sickness  had  to  be  overcome.  But  on  July  28, 
the  weather  conditions  along  the  eclipse  track  were  of  the 
very  best. 

The  first  change  noted  in  the  corona  of  1878  was  the 
enormous  decrease  in  total  lustre,  when  compared  with  the 
coronas  of  1870  and  1871,  Harkness  estimating  the 


152  ECLIPSES    OF    THE    SUN 

luminosity  to  be  only  one-seventh  of  the  corona  of  1870, 
while  Lockyer  regarded  1878  to  be  one- tenth  of  the  bright- 
ness of  the  1871  eclipse.  The  decrease  in  brilliancy  was 
accompanied  by  a  remarkable  and  unexpected  change  in 
shape.  To  Janssen  in  1871,  the  dark  moon  looked  like  the 
center  of  a  giant  dahlia,  the  corona  being  nearly  circular  in 
outline.  In  1878,  the  streamers  along  the  sun's  axis  were 
much  shorter  in  length  but  much  more  pronounced  in  char- 
acter, these  polar  rays  resembling  more  than  anything  else 
the  lines  of  force  around  a  magnet.  But  the  most  astounding 
phenomenon  was  the  enormous  extent  of  the  coronal  stream- 
ers along  the  sun's  equator.  Langley  in  the  pure,  rare  air 
of  Pike's  Peak  followed  these  streamers  to  six  diameters  of 
the  moon  on  one  side,  but  on  the  other  side  where  he  had 
been  more  intently  watching,  to  the  colossal  length  of  twelve 
diameters,  or  more  than  ten  millions  of  miles!  These 
equatorial  extensions  were  confirmed  by  Simon  Newcomb  in 
Separation,  Wyoming,  by  Cleveland  Abbe  farther  down  the 
slope  of  Pike's  Peak,  and  by  almost  every  astronomer  who 
witnessed  the  eclipse.  The  perplexities  surrounding  the 
corona  were  accordingly  multiplied  many-fold,  for  how- 
could  a  solar  atmosphere  obeying  gravity  exist  at  the  huge 
distance  of  ten  million  miles  from  the  sun's  surface?  Young 
and  Abbe  saw  long  faint  beams  shining  along  the  sun's 
axis. 

Remarkable  as  were  the  visual  phenomena  manifested, 
their  testimony  was  no  whit  stranger  than  the  revelations 
by  means  of  the  spectroscope.  The  hydrogen  and  green 
coronium  emission  lines  were  visible,  but  with  such  vastly 
diminished  intensity  compared  with  the  eclipse  of  1871  that 
most  observers  completely  missed  seeing  them.  They  were, 
however,  visible  to  Young,  Eastman  and  some  others.  If 
the  emission  lines  were  weak,  the  Fraunhofer  lines  of  the 
corona  were  comparatively  strong,  showing  that  the  re- 
flected light  near  the  sun's  limb  was  relatively  stronger  than 
in  1871,  a  fact  confirmed  by  observations  with  the  polari- 
scope. 

The  eclipse  of  1878  showed  the  long  equatorial  wings 
of  the  corona,  strong  polar  brushes,  faint  incandescent  light 


ECLIPSES    AFTER    1878  153 

of  coronium  and  hydrogen,  and  light  reflected  strongly 
from  material  particles  near  the  sun's  limb.  Were  each  of 
these  four  special  features  unalterably  connected  with  the 
condition  of  the  sun-spot  minimum,  or  did  they  happen 
merely  by  chance?  Time  alone  could  furnish  the  answer. 
Great  progress  was  made  at  this  eclipse  in  the  photography 
of  the  corona,  particularly  by  the  use  of  portrait  lenses  which 
were  successful  in  portraying  a  mass  of  detail  in  the  inner 
and  brighter  corona,  but  failed  to  show  the  outer  streamers. 
Photographs  of  these  faint  extensions  must  needs  wait 
until  some  date  in  the  future  when  plates  of  greater  sensi- 
tivity could  be  produced. 

Another  important  observation  at  the  eclipse  of  1878  was 
the  discovery  (?)  of  two  bright  star-like  objects  by  two 
American  astronomers,  Swift  and  Watson.  The  objects 
could  not  be  identified  with  any  of  the  fixed  stars,  and  it 
was  therefore  necessarily  assumed  that  they  were  small 
planets  moving  about  the  sun  inside  of  the  orbit  of  Mer- 
cury. The  reputations  of  these  two  astronomers  for  care- 
ful observing  were  so  great  that  it  cost  the  science  of  as- 
tronomy a  quarter  of  a  century  of  eclipse  observations 
before  it  was  finally  decided  that  no  intra-Mercurial  planets 
exist  which  are  as  large  or  as  bright  as  the  objects  supposed 
to  have  been  seen. 

The  next  eclipse  to  be  observed  was  that  of  May  17, 
1882,  the  forerunner  in  the  Saros  of  the  eclipse  of  May  28, 
1900.  The  1882  eclipse  was  seen  in  Egypt  with  a  brief 
duration  of  totality  amounting  to  seventy-four  seconds. 
This  eclipse  is  memorable  on  account  of  the  bright  comet 
that  was  seen  and  photographed  near  the  sun,  the  comet 
not  being  observed  either  before  or  after  the  eclipse.  The 
photographic  plates  had  now  become  more  rapid,  the  dry 
plate  having  been  invented,  and  accordingly  the  astrono- 
mers had  to  their  hands  better  facilities  for  attacking  the 
corona  with  camera  and  spectroscope.  Also  for  the  first 
time  we  hear  of  the  prismatic  camera,  which  is  a  slitless 
spectroscope,  with  a  photographic  plate  to  take  the  place 
of  the  observing  eye-piece;  and  this  instrument,  particu- 
larly in  the  hands  of  Lockyer,  was  to  play  an  important  role 


154  ECLIPSES    OF    THE    SUN 

in  eclipse  spectroscopy.  Eleven  years  having  elapsed  since 
1871,  the  year  1882  was  one  of  maximum  sun-spots,  there 
being  no  less  than  twenty-three  separate  spots  on  the  face 
of  the  sun  on  the  day  before  the  eclipse.  The  form  of  the 
corona  in  no  way  resembled  that  of  the  minimum  of  1878 
but  bore  a  striking  resemblance  to  the  crown  of  glory  of 
1871,  the  shape  being  more  nearly  rectangular ,  or  even  star- 
like,  and  the  long  equatorial  extensions  and  strong  polar 
brushes  being  entirely  lacking.  The  spectroscope  also  re- 
vealed vast  differences  from  the  eclipse  of  1878.  The  corona 
as  a  whole  was  more  brilliant  than  that  of  the  preceding 
eclipse,  and  the  emission  lines  of  the  spectrum  were  obtained 
both  by  a  slit  spectroscope  and  by  the  prismatic  camera. 
With  the  former  instrument,  Schuster  photographed  about 
thirty  lines  in  the  spectrum  of  the  corona.  Many  new  spec- 
tral lines  were  visible  in  the  red  and  violet  to  Tacchini  and 
Thollon  respectively.  These  lines  were  seen  and  photo- 
graphed during  the  progress  of  totality,  and  not  near  the 
beginning  or  end  of  the  total  phase.  Apparently  the  lines 
did  not  seem  to  belong  to  the  flash  spectrum  and  must  have 
their  origin  in  the  true  corona.  But  for  the  first  time  a  sus- 
picion seems  to  have  been  aroused  that  the  lines  might  after 
all  be  due  to  prominences  and  chromosphere  and  not  to  the 
true  corona,  for  Schuster  observed  the  H  and  K  lines  of  cal- 
cium to  appear  bright  even  across  the  face  of  the  dark  moon 
where  no  light  at  all  was  supposed  to  exist!  Evidently  the 
chromospheric  light  was  reflected  by  some  atmosphere 
somewhere,  either  in  the  higher  reaches  of  the  sun's  atmos- 
phere directly  in  line  with  the  center  of  the  dark  moon, 
or  in  the  atmosphere  of  the  earth.  The  first  condition  could 
hardly  be  possible  and  that  left  no  contingency  other 
than  the  second.  Schuster's  observation  was  not  the  first 
to  reveal  bright  lines  on  the  dark  face  of  the  moon  because 
as  early  as  1870  Young  had  perceived  bright  hydrogen  lines. 
If  the  lines  of  emission  were  stronger  in  1882  than  in 
1878,  it  was  not  so  with  the  dark  Fraunhofer  lines.  The 
spectroscopes  revealed  the  continuous  spectrum  in  the 
brighter  inner  corona,  but  farther  from  the  sun  the  dark  lines 
due  to  reflected  photospheric  light  were  observed  both  visu- 


ECLIPSES    AFTER    1878  155 

ally  and  in  the  photographs,  but  these  lines  were  not  so  strong 
as  in  1878.  Sun-spot  maximum  appeared  therefore  to 
correspond  to  a  star-like  corona,  with  no  polar  brushes, 
strong  coronium  and  other  bright  coronal  lines,  but  with  the 
Fraunhofer  lines  intrinsically  weaker  than  at  sun-spot 
minimum. 

The  direct  photographs  of  the  corona  by  Schuster  were 
in  better  definition  and  showed  more  details  than  those  of 
previous  eclipses.  In  fact,  the  impressions  made  on  the 
plates  were  so  strong  that  Dr.  Huggins  obtained  the  idea 
that  it  might  even  be  possible  to  photograph  the  corona 
without  an  eclipse.  For  observing  the  prominences  without 
waiting  for  an  eclipse,  the  spectroscope  had  already  been 
utilized  to  get  rid  of  the  glare  of  the  sunlight  in  our  own 
atmosphere.  It  is  not  possible  to  make  use  of  the  spectro- 
scope in  the  same  manner  for  obtaining  coronal  photographs, 
for  the  simple  reason  that  the  bright-line  radiations  of  coro- 
nium are  not  sufficiently  strong  in  character  to  enable  the 
coronium  light  to  shine  in  contrast  with  the  enfeebled  solar 
glare.  Many  attempts  to  photograph  the  corona  in  full  sun- 
light were  made  by  a  variety  of  different  methods  lasting 
over  a  number  of  years,  the  astronomers  being  urged  on  by 
great  hopes  since  the  first  trial  photographs  seemed  to  pre- 
dict success.  The  details  of  the  early  work  of  Huggins,  and 
the  later  researches  will  be  deferred  until  a  subsequent 
chapter. 

One-third  of  the  way  round  the  globe  eastwards  from  the 
Dutch  East  Indies,  where  the  eclipse  of  1901,  May  18,  was 
observed,  brings  one  to  the  middle  of  the  Pacific  Ocean. 
The  eclipse  track  of  May  6,  1883,  lay  almost  entirely  across 
the  water,  but  fortunately,  in  its  path  there  was  a  small  coral 
reef,  only  seven  miles  long,  and  unknown  ten  years  previ- 
ously. The  importance  of  the  discoveries  of  1882  and  the 
fact  that  the  eclipse  of  the  year  1883  was  of  the  very  long 
duration  of  more  than  five  minutes,  attracted  to  Caroline 
Island,  astronomers  from  America,  England,  France,  Austria 
and  Italy.  The  great  risks  taken  by  eclipse  expeditions  in 
the  tropics  of  being  overtaken  by  clouds  was  shown  in 
this  eclipse,  but  fortunately  a  clear  spell  between  two 


156  ECLIPSES    OF    THE    SUN 

periods  of  clouds  was  experienced.  The  general  features 
of  the  corona  greatly  resembled  that  of  the  year  before,  the 
sun  continuing  to  show  many  spots.  Owing  to  the  long  ex- 
posures possible,  excellent  photographs  of  the  corona  were 
secured,  and  for  the  first  time  in  the  history  of  eclipses, 
greater  extensions  of  streamers  were  photographed  than 
were  visible  to  the  eye. 

The  most  important  observations  were  unquestionably  by 
means  of  the  spectroscope.  Up  to  this  time,  all  observa- 
tions during  totality  had  shown  a  continuous  spectrum  of 
the  corona  close  to  the  sun,  and  farther  out  faint  Fraun- 
hofer  lines,  with  the  bright  green  line  of  coronium  crown- 
ing the  whole.  According  to  the  dissociation  theory  of 
Lockyer,  neither  continuous  spectrum  nor  dark  lines  could 
exist  there,  and  "  if  these  statements  regarding  the  corona 
were  strictly  accurate  my  hypothesis  was  worthless/'1 
Hence,  a  careful  search  was  made  by  Janssen  for  Fraun- 
hofer  lines.  They  were  found  by  him  in  great  numbers, 
thus  confirming  the  observation  of  1882.  To  make  as- 
surance doubly  sure,  the  dark  spectrum  lines  in  the  corona 
were  successfully  photographed.  As  a  result  of  these  ob- 
servations, Janssen  concluded  2  that  "  the  basis  of  the  coro- 
nal spectrum  was  formed  by  the  complete  Fraunhofer  spec- 
trum, and  that,  therefore,  there  exists  in  the  corona,  and 
above  all  in  certain  localities  of  it,  an  enormous  amount  of 
reflected  light;  and  since  we  know  that  the  coronal  atmos- 
phere is  very  rare,  it  follows  that  these  regions  must 
abound  in  cosmic  matter  in  the  state  of  solid  corpuscles,  in 
order  to  explain  the  abundance  of  reflected  sunlight." 

Spectroscopic  observations  of  great  interest  were  made 
on  the  corona  by  Hastings.  He  used  a  60°  prism  attached 
to  a  six-inch  telescope,  there  being  two  totally  reflecting 
prisms  placed  outside  the  slit  so  that  the  spectrum  of  two 
opposite  sides  of  the  sun  could  be  brought  together  and  ex- 
amined by  comparison.  The  observations  were  confined  to 
the  green  coronium  line.  At  the  beginning  of  totality,  this 
line  was  12'  in  length  and  very  bright  on  the  eastern 

1  Lockyer,  Chemistry  of  the  Sun,  p.  .365. 

2  Comptes  Rendus,  92,  No.  10. 


ECLIPSES    AFTER    1878  157 

limb,  while  on  the  western  limb  it  was  only  4'  in  length  and 
comparatively  faint.  As  the  eclipse  advanced ,  the  inequal- 
ity vanished,  at  mid-totality  conditions  were  equal,  while 
at  the  end  of  totality  the  lines  on  the  western  limb  were  the 
longer  and  brighter.  Such  a  great  change  could  not  be  ex- 
plained by  assuming  that  the  moon  in  its  motion  progres- 
sively covered  and  uncovered  the  bright  coronal  radiations, 
and  accordingly,  Hastings  attempted  to  explain  his  observa- 
tions on  the  assumption  that  the  outer  corona  has  no  real 
existence  but  that  its  appearance  is  caused  by  diffraction 
round  the  edge  of  the  moon.  On  this  hypothesis,  the  true 
corona  is  confined  to  a  very  narrow  ring  around  the  sun, 
the  light  from  this  inner  ring  of  material  substance  being 
widened  by  diffraction  to  form  the  outer  corona  which  thus 
takes  upon  itself  all  of  the  appearances  of  reality.  To  the 
astronomers  who  had  seen  the  great  extensions  of  1878,  it 
was  hard  to  believe  that  diffraction  of  light  could  adequately 
explain  the  detail  of  the  coronal  streamers  at  the  great  dis- 
tance of  twelve  diameters  from  the  sun's  limb,  but  it  was 
equally  difficult  to  understand  how  luminescence  could 
exist  in  a  solar  atmosphere  at  the  colossal  distance  of  ten 
million  miles  from  the  sun's  surface.  If  the  coronal  light 
were  reflected,  it  could  not  be  seen  unless  reflected  from 
material  particles,  and  if  the  light  were  intrinsic,  how  could 
it  have  any  existence  in  an  atmosphere  so  infinitesimally 
rare?  The  answer  given  by  Hastings  denied  the  solar  origin 
of  the  corona,  and  seemed  to  be  a  step  backward.  Appar- 
ently there  was  no  way  out  of  the  quandary,  but  to  wait  for 
future  eclipses. 

In  the  attempt  to  secure  information  regarding  the  flash 
spectrum  three  distinct  improvements  in  the  line  of  attack 
were  inaugurated  in  1882:  i.  Eye  observations  were  no 
longer  to  be  trusted  exclusively,  the  spectrum  must  be 
photographed.  2.  A  grating  was  used,  before,  during  and 
after  totality.  3.  A  moving  plate  was  utilized  with  an 
integrating  prism  spectroscope  to  secure  photographs  before, 
during  and  after  totality.  The  grating  showed  little  in  ad- 
dition to  the  H  and  K  lines  seen  near  the  limb  throughout 
totality.  The  photographs  attempting  to  secure  the  revers- 


158  ECLIPSES    OF    THE    SUN 

ing  layer  succeeded  only  in  imprinting  the  hydrogen  lines 
and  comparatively  few  of  the  brighter  lines,  in  fact  the 
total  number  of  lines  were  certainly  too  deficient  to  guar- 
antee the  confirmation  of  Young's  belief  that  the  whole 
Fraunhofer  spectrum  was  reversed  in  the  flash  spectrum. 

The  total  eclipse  of  August  29,  1886,  was  visible  in  the 
West  Indies.  The  energies  of  many  of  the  observers  were 
devoted  to  testing  the  method  of  Huggins  of  photographing 
the  corona  without  an  eclipse.  For  this  purpose,  fifteen 
separate  photographs  were  taken  on  the  day  before  the 
eclipse  and  a  series  of  twenty  during  the  partial  phases, 
these  photographs  to  be  compared  with  plates  obtained 
during  totality.  The  conclusions  were  quite  definite,  for 
not  a  single  one  of  the  coronal  details  was  found  on  the 
plates  taken  outside  of  totality,  and  it  seemed  therefore  nec- 
essary to  decide  that  it  was  impossible  to  photograph  the 
corona,  except  within  the  limits  of  a  total  eclipse,  at  least 
under  the  conditions  of  hazy  sky  and  low  sun  that  had  pre- 
vailed. 

Tacchini  made  a  careful  comparison  of  the  prominences 
observed  spectroscopically  before  and  after  totality  with 
those  seen  directly  during  the  total  phase,  and  he  concluded 
that  all  the  prominences  showed  themselves  larger  and 
taller  during  an  eclipse,  the  upper  portions  being  white  in 
color  when  the  prominences  exceeded  i'  of  arc  in  height. 
The  differences  of  apparent  height  may  find  a  ready  ex- 
planation in  the  effect  of  contrast  with  the  background,  in- 
side and  outside  of  totality,  but  the  matter  of  the  color  of 
the  prominences  could  not  be  so  readily  settled.  For  many 
years  "  white  "  prominences  found  a  conspicuous  place  in 
spectroscopic  literature.  Tacchini  observed  the  flash 
spectrum  visually.  Turner  attempted  to  observe  changes 
in  the  coronal  streamers  resembling  currents,  but  obtained 
no  results  of  value. 

One  of  the  expeditions  at  this  eclipse  experienced  a  series 
of  accidents  due  to  the  unavoidable  necessity  of  employing 
volunteer  observers  as  assistants.  Some  of  the  incidents 
might  have  been  amusing  if  an  opportunity  had  been  af- 
forded on  the  morrow  for  making  another  attempt,  —  but 


ECLIPSES    AFTER    1878  159 

at  the  rare  event  of  a  total  eclipse  such  untoward  happenings 
are  not  laughing  matters,  but  become  almost  tragedies. 
Some  of  these  mishaps  were:  failure  to  get  the  sun's  image 
on  the  photographic  plate  in  the  most  important  instrument, 
the  breaking  of  the  polar  axis  just  before  totality,  in  the 
next  important  instrument,  the  failure  of  an  assistant  to 
make  exposures,  the  standing  of  two  native  policemen  in 
front  of  the  photometer  during  totality,  the  seizure  of  the 
plates  by  well-meaning  but  ill-advised  customs  officials, 
thereby  causing  a  delay  in  the  development  of  the  plates 
until  the  following  May  by  which  time  the  plates  had 
greatly  deteriorated.  Eclipse  observers  generally  are  not 
anxious  to  repeat  such  a  chapter  of  accidents. 

The  eclipse  of  the  following  year,  August  19,  1887,  was 
one  of  widespread  disappointment,  for  the  projects  so  care- 
fully prepared  ended  only  in  failure  to  secure  results,  not 
through  any  fault  in  the  plans  themselves  but  on  account  of 
the  astronomer's  enemy,  clouds,  which  prevailed  almost 
everywhere.  Fine  weather,  due  to  holes  in  the  cloudy  sky, 
prevailed  at  several  of  the  stations  in  Russia  and  Japan, 
however,  and  some  photographs  and  observations  of  value 
were  secured. 

The  year  1889  brought  two  eclipses,  both  extensively  ob- 
served. Here  was  inaugurated  the  splendid  series  of  ex- 
peditions sent  out  from  the  Lick  Observatory.  The  path  of 
the  eclipse  of  January  i  crossed  Nevada  and  California,  and 
the  photographers  near  the  line  of  totality  were  so  well  or- 
ganized for  the  work  by  Mr.  Charles  Burckhalter  that  an 
excellent  series  of  photographs  of  the  corona  resulted. 
The  best  photograph  secured  at  this  eclipse,  and  in  fact  the 
very  best  obtained  at  any  eclipse  to  this  date,  was  secured 
by  Barnard.  The  equipment  was  very  meager.  The  largest 
lens  employed  was  3^  inches  aperture,  stopped  down  to  i^ 
inches,  and  of  49  inches  focus.  Barnard's  success  depended 
on  an  accurate  adjustment  of  the  instrument  but  more 
specially  on  the  skill  and  care  with  which  the  plates  were 
developed.  Barnard  was  a  professional  photographer  be- 
fore he  was  an  astronomer,  and  he  was  thoroughly  familiar 
with  the  best  methods  of  developing  a  plate  in  order  to 


160  ECLIPSES    OF    THE    SUN 

bring  forth  all  of  the  latent  detail.  The  eclipse  of  Decem- 
ber 22  of  the  same  year  was  successfully  photographed  at 
Cayenne  in  the  West  Indies  by  the  Lick  party  consisting 
of  Burnham  and  Schaeberle.  The  photographs  showed  that 
changes  had  occurred  in  the  corona  since  the  eclipse  of  the 
beginning  of  the  year.  The  earlier  eclipse  took  place  near 
sun-spot  minimum  and  exhibited  the  equatorial  extensions 
of  the  corona.  The  eclipse  of  December  22  is  memorable 
from  the  death  of  Father  Perry  a  few  days  after  the  eclipse, 
a  martyr  to  the  cause  of  science.  This  brave  man,  though 
greatly  weakened,  took  part  in  the  eclipse  work,  and  having 
found  as  soon  as  totality  passed  that  everything  had  passed 
off  well,  he  called  for  three  hearty  British  cheers,  —  which 
unfortunately  he  could  not  himself  lead. 

The  greatest  success  attended  the  observations  of  April 
1 6,  1893,  largely  through  the  use  of  apparatus  much  more 
powerful  than  had  ever  been  employed  before  at  an  eclipse. 
The  most  conspicuous  advance  came  to  the  party  from  the 
Lick  Observatory  in  Chile  who  used  a  camera  of  five  inches 
aperture  and  forty  feet  focus  for  securing  photographs  of 
the  corona.  Schaeberle  decided  to  point  the  objective  di- 
rectly at  the  sun  and  to  mount  it  on  one  fixed  pier  and  the 
movable  photographic  plate  on  another,  both  piers  to  be 
wholly  free  from  contact  with  the  great  tube  extending 
between  lens  and  plate.  The  slide  carrying  the  photo- 
graphic plate  was  the  only  moving  part,  and  its  motion  was 
so  regulated  by  means  of  inclined  planes  as  to  give  it  the 
same  velocity  and  direction  as  the  sun's  focal  image  during 
the  eclipse.  The  details  of  erecting  this  instrument,  known 
as  the  Schaeberle  mounting,  are  found  in  the  Contributions 
from  the  Lick  Observatory,  No.  4.  A  careful  focus  was 
secured  and  beautiful  photographs  were  obtained  showing 
the  prominences  and  inner  corona  with  a  definition  which 
left  little  to  be  desired. 

Optical  power,  up  to  then  unprecedented,  was  employed  in 
the  prismatic  camera  designed  by  Lockyer  and  used  by 
Fowler  in  West  Africa.  The  camera  had  a  focal  length  of 
7  feet  6  inches  with  a  prism  of  45°,  giving  a  dispersion  of 
about  two  inches  from  F  to  K.  Fowler  secured  photographs 


ECLIPSES    AFTER    1878  161 

which  were  supplemented  by  a  series  obtained  by  Shackleton 
in  Brazil,  with  the  result  that  the  positions  of  164  chromo- 
spheric  lines  were  measured  between  F  and  K.  Deslandres, 
at  the  same  eclipse,  attempted  to  measure  the  rate  of  rotation 
of  the  corona  by  observing  the  relative  displacement  of  the 
spectra  of  two  regions  of  the  corona  at  opposite  sides  of 
the  sun  placed  in  juxtaposition.  A  grating  spectroscope 
was  used,  and  the  conclusion  reached  was  that  the  corona 
partakes  of  the  general  rotation  of  the  sun.  Unfortunately, 
there  is  no  justification  for  this  deduction  by  Deslandres 
since  the  measures  made  by  him  were  on  the  H  and  K  lines 
which  belong  to  the  chromosphere  and  not  to  the  corona. 
One  of  the  most  important  results  of  this  eclipse  was  that 
it  became  possible  for  the  first  time  in  eclipse  spectroscopy 
to  separate  clearly  the  spectrum  of  the  corona  from  that 
of  the  chromosphere,  and  it  was  henceforth  no  longer  as- 
sumed that  a  spectral  line  visible  during  totality  belonged 
of  necessity  to  the  corona. 

The  eclipse  of  1896,  taking  place  on  August  9,  was  ob- 
served by  a  large  number  of  expeditions.  An  English  party 
consisting  of  Christie,  Turner  and  Hills  went  to  Japan 
where  also  was  one  from  the  Lick  Observatory  and  another 
American  expedition  headed  by  Todd;  and  also  two  Japa- 
nese parties. 

Lockyer  went  in  H.  M.  S.  Volage  to  Norway  where 
a  large  party  of  seventy-five,  including  officers  and  sailors 
of  the  ship,  took  care  of  a  large  and  varied  program.  In 
one  department  of  the  work,  for  instance,  in  the  sketching 
of  the  corona,  a  competition  was  started  on  board  ship  by 
thirty-five  volunteer  sketchers,  an  artificial  corona  being  ex- 
posed to  view  for  105  seconds,  the  time  of  duration  of 
totality.  Sixteen  who  showed  the  greatest  proficiency  were 
selected  for  sketching  of  the  corona  on  eclipse  day.  But 
alas,  for  "  the  best  laid  schemes  o'  mice  and  men,"  —  the 
clouds  prevailed  almost  everywhere  except  where  there  was 
a  small  English  party  consisting  of  Stone  and  Shackleton. 
The  latter  was  successful  in  timing  his  observations  with 
the  prismatic  camera  so  well  that  the  long  desired  photo- 
graph of  the  flash  spectrum  was  at  last  secured.  Although 


162  ECLIPSES    OF    THE    SUN 

the  focus  was  not  of  the  very  finest,  still  there  were  shown  a 
total  of  464  lines  in  the  spectrum  between  F  and  K.  This 
photograph,  taken  by  one  of  Lockyer's  assistants,  sounded 
the  death-knell  of  the  dissociation  theory,  —  but  Lockyer 
still  refused  to  be  convinced  that  the  flash  spectrum  was  a 
reversal  of  the  Fraunhofer  spectrum.  His  argument  was  a 
very  simple  one,  which  was,  that  between  F  and  K,  5694 
Fraunhofer  lines  were  tabulated  by  Rowland,  while  in  the 
eclipse  spectra  of  1893  there  were  but  164  lines  and  in  1896 
but  464  lines,  consequently  showing  but  three  and  eight 
percent,  respectively,  of  the  Fraunhofer  lines  reversed  in 
the  flash  spectrum.  Lockyer  however  failed  to  draw  atten- 
tion to  the  fact  that  Rowland's  atlas  was  secured  with  a 
much  greater  dispersion  than  that  used  at  the  eclipse  and 
with  vastly  superior  definition.  Lockyer's  conclusion,1  the 
result  of  the  spectra  at  these  two  eclipses,  was  that  "  the 
chromosphere  is  a  region  of  high  temperature  in  which  there 
is  a  corresponding  simplification  of  spectrum  as  compared 
with  the  cooler  region  in  which  the  Fraunhofer  absorption 
is  produced."  The  manner  of  settling  the  question,  the  way 
of  advancement  for  future  eclipses  was  clearly  indicated: 
the  flash  spectrum  must  again  be  photographed  and  with 
increased  dispersion,  and  great  care  should  be  exercised  to 
see  that  the  exposures  were  made  at  the  correct  times,  with 
as  good  focus  and  definition  as  possible. 

Such  photographs  were  secured  at  the  eclipse  of  January 
22,  1898,  visible  in  India,  where  such  excellent  conditions  of 
weather  were  experienced  that  then  partially  compensated 
for  the  ill  luck  of  the  previous  eclipse.  The  largest  expedi- 
tion in  point  of  numbers  was  that  under  the  direction  of  Sir 
Norman  Lockyer  located  at  Viziadrug  on  the  West  Coast, 
the  astronomers  being  assisted  by  the  officers  and  men  of 
H.  M.  S.  Melpomene.  The  program  was  an  extensive  one, 
embracing  visual  and  photographic  observations  of  the 
corona,  the  most  important  problem  being  a  spectroscopic 
attack  on  the  chromosphere  with  two  large  prismatic  cameras 
of  six  and  nine  inches  aperture.  By  these  two  instruments 
about  sixty  photographs  were  secured,  the  exposure  times 

1  Recent  and  Coming  Eclipses,  p.  in. 


ECLIPSES    AFTER    1878  163 

varying  from  i  to  59  seconds.  These  included  two  series  of 
ten  snap-shots  at  the  beginning  and  another  ten  at  the  end  of 
totality  and  a  number  of  exposures  of  different  lengths  dur- 
ing totality.  Christie  and  Turner,  representing  the  British 
Joint  Permanent  Eclipse  Committee,  were  at  Sahdol.  Cope- 
land  was  at  Goglee,  Newall  and  Hills  were  at  Pulgaon, 
Campbell  of  the  Lick-Crocker  expedition  was  at  Jeur,  while 
at  Talni  was  located  Evershed  and  also  Mr.  and  Mrs. 
Maunder. 

The  most  important  problem  was  that  of  the  flash  spec- 
trum, and  fortunately,  successful  photographs  were  secured 
by  Fowler  and  Dr.  Lockyer,  by  Campbell,  by  Hills,  by 
Newall  and  by  Evershed..  A  discussion  of  the  spectra  by  Sir 
N.  Lockyer  again  confirmed  him  in  the  opinion  he  had  held 
since  1873,  that  many  strong  chromospheric  lines  were  not 
represented  among  the  Fraunhofer  lines,  while  many  of  the 
dark .  lines  found  under  ordinary  conditions  in  the  solar 
spectrum  did  not  appear  as  bright  lines  in  the  flash  spectrum. 
He  therefore  concluded  that  the  flash  did  not  represent 
the  spectrum  of  the  reversing  layer.  It  is  true  that  the  hy- 
drogen series  and  the  helium  lines  of  the  flash  spec- 
trum are  not  found  in  the  Fraunhofer  spectrum,  and  also 
that  there  are  great  differences  in  intensity  between  the  two 
spectra,  and  in  a  sense  therefore  one  spectrum  is  not  the 
exact  reversal  of  the  other;  but  none  the  less,  it  is  impossible 
to  reach  any  conclusion  other  than  that  practically  every 
strong  dark  line  in  the  solar  spectrum  is  present  as  a  bright 
line  in  the  flash  spectrum.  The  matter  will  be  treated  more 
fully  in  Chapter  XVI.  The  excellent  photographs  obtained 
by  Fowler  and  Lockyer  permitted  a  discovery  of  the  greatest 
value  to  future  scientific  development,  namely,  that  "  en- 
hanced "  lines,  or  those  which  are  relatively  stronger  in  the 
spectrum  of  the  spark  than  in  the  arc,  are  specially  promi- 
nent in  the  spectrum  of  the  chromosphere. 

Exquisite  photographs  of  the  corona  with  the  4O-foot 
camera  were  secured  by  Campbell,  while  Mrs.  Maunder 
with  a  Dallmeyer  lens  of  only  one  and  a  half  inches  aper- 
ture photographed  the  faint  extensions  of  the  corona  run- 
ning out  to  nearly  six  diameters  from  the  moon's  limb.  The 


164  ECLIPSES    OF    THE    SUN 

corona  of  1898  presented  a  mixed  aspect,  a  combination 
of  the  polar  brushes  observable  at  sun-spot  minimum  being 
combined  with  the  quadrilateral  shape  of  sun-spot  maxi- 
mum. 

One  contribution  of  great  importance  was  the  measure- 
ment of  the  wave-length  of  the  green  coronium  line.  For 
nearly  thirty  years  since  its  discovery,  it  had  been  assumed 
that  the  coronium  line  was  identical  in  position  with  the 
chromospheric  line  at  5316.8  A.  Lockyer,  Fowler,  Evershed 
and  Campbell  independently  found  the  coronium  line  to  be 
farther  to  the  violet,  at  5303  A.  That  the  value  of  this 
wave-length  was  now  found  for  the  first  time,  in  spite  of 
observations  made  at  several  eclipses,  will  show  more  clearly 
than  words  can  express  that  the  eclipse  spectra  prior  to  1898 
were  poor  in  definition  and  small  in  dispersion. 

Newall  used  a  spectrograph  with  two  slits  with  which  he 
hoped  to  secure  photographs  to  test  the  rotation  of  the 
corona.  Unfortunately,  the  slits  were  placed  8'  from  the 
sun's  limb  and  the  coronal  light  was  too  feeble  to  impress 
any  traces  on  the  plate.  Newall  observed  the  corona  with 
a  polariscope  while  Turner  attempted  to  achieve  similar  ob- 
servations by  photography. 

A  new  epoch  of  accuracy  in  photographing  the  chromo- 
spheric spectrum  having  been  begun  in  1898,  it  was  but 
natural  that  every  effort  should  be  made  to  continue  the 
success  of  this  work  in  1900,  in  order  to  secure,  if  possible, 
still  greater  definition  with  larger  dispersion.  The  eclipse 
track  of  May  28,  1900,  lay  over  the  southeastern  part  of 
the  United  States,  and  after  crossing  the  Atlantic  Ocean, 
passed  over  Portugal,  Spain  and  Algeria.  On  account  of  its 
easy  accessibility  to  American  and  European  astronomers 
the  eclipse  was  witnessed  by  a  greater  number  of  observers 
than  ever  before  in  the  history  of  eclipses.  Fortunately 
good  weather  was  experienced  almost  everywhere.  The 
program  was  a  wide  and  varied  one,  and  it  will  be  possible 
here  to  mention  only  a  few  special  lines  of  work  and  record 
the  names  of  comparatively  few  of  the  many  observers. 

Photographs  of  the  corona  were  taken  in  numbers  to  the 
hundreds,  or  even  to  the  thousands,  by  small,  medium, 


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ECLIPSES    AFTER    1878  165 

large  and  huge  sized  cameras,  the  greatest  focal  length  be^ 
ing  that  employed  by  the  party  from  the  Smithsonian  In- 
stitution who  utilized  a  lens  of  twelve  inches  aperture  and 
135  feet  focal  length.  The  photographs  on  this  large  scale 
were  in  good  definition  and  they  showed  a  great  wealth  of 
detail  in  the  inner  corona.  Excellent  photographs  were  se- 
cured at  Wadesboro,  N.  C.,  by  Professor  Barnard  and  Mr. 
Ritchey  with  a  horizontal  camera  of  61^  feet  focus.  A  re- 
production of  the  exposure  of  thirty  seconds  is  given  on 
page  1 64 .  This  eclipse  having  taken  place  near  spot-minimum 
shows  strong  polar  brushes  and  long  equatorial  streamers. 

Both  in  America  and  in  Europe,  an  extended  attack  was 
made  on  the  flash  spectrum  by  slit  spectroscopes,  by  pris- 
matic cameras  and  by  gratings,  plane  and  concave,  used  both 
with  and  without  a  slit.  The  United  States  Naval  Observa- 
tory had  three  stations  in  the  field,  at  Barnesville,  Ga.,  at 
Griffin,  Ga.,  and  at  Pinehurst,  N.  C.  The  three  concave 
gratings  used  by  Ames,  Crew,  and  Humphreys  were  each 
employed  with  a  slit.  The  photographs  showed  nothing. 
With  plane  grating  and  quartz  lens,  Huff  secured  a  well-ex- 
posed spectrum,  —  but  the  focus  was  not  of  the  very  best. 
Better  results  were  secured  by  Frost  who  used  a  concave 
grating,  objectively  without  slit. 

In  Europe,  Sir  Norman  Lockyer,  located  at  Santa  Pola 
in  Spain,  carried  out  an  extended  series  of  observations 
similar  in  scope  to  those  made  at  Viziadrug  in  1898.  The 
nine-inch  prism  was  combined  with  a  camera  20  feet  in 
focal  length  in  order  to  secure  a  great  linear  dispersion  in 
the  resulting  photographs  for  the  purpose  of  measuring  the 
heights  of  the  various  layers  forming  the  sun's  chromo- 
sphere. Successful  photographs  of  the  flash  spectrum  were 
secured  by  Fowler  and  Dr.  Lockyer,  and  also  by  Dyson  at 
Ovar,  and  Evershed  at  Mazapan  in  Algeria.  The  latter 
station  was  selected  so  that  it  might  be  as  near  as  pos- 
sible to  the  edge  of  the  band  of  totality  so  that  the  photo- 
graphs of  the  chromosphere  might  be  obtained  in  high 
solar  latitudes.  Unfortunately,  through  an  error  in  the 
Nautical  Almanac,  Evershed  found  himself  just  outside,  in- 
stead of  barely  inside,  the  path  of  totality.  The  series  of 


166  ECLIPSES    OF    THE    SUN 

photographs  obtained,  however,  were  of  fine  definition  and 
were  specially  valuable  in  affording  a  means  of  comparison 
with  photographs  of  the  flash  spectrum  which  have  usually 
been  taken  near  the  solar  equator.  This  comparison  shows 
that  the  spectrum  of  the  chromosphere  is  the  same  at  the 
sun's  polar  regions  as  at  low  latitudes,  and  it  appears  fairly 
certain  that  the  spectrum  of  the  sun's  limb  is  as  constant 
in  character  as  the  ordinary  Fraunhofer  spectrum.  In  this 
connection  it  should  be  borne  in  mind  that  Evershed's  photo- 
graphs showed  the  flash  spectrum  where  the  moon  was  prac- 
tically at  grazing  incidence  with  the  sun,  and  consequently 
the  layer  of  the  chromosphere  photographed  must  have  been 
very  close  to  the  edge  of  the  photosphere. 

Fortunately,  the  Algiers  Observatory  was  in  the  shadow 
track,  and  here  through  the  kindness  of  the  director,  M. 
Trepeid,  Mr.  Wesley  was  given  an  opportunity  to  observe 
the  corona  visually  with  an  equatorial  coude  of  0.3  meters 
aperture.  Mr.  Wesley's  long  experience  in  the  study  of  the 
corona  had  well  fitted  him  for  the  task  of  finding  out  whether 
more  detail  could  be  observed  visually  than  is  portrayed  on 
the  best  photographs.  The  conclusion  reached  was  very 
definite  that  the  photograph,  if  on  a  sufficiently  large  scale, 
exhibits  all  of  the  coronal  features  that  can  be  seen  with 
the  eye. 

Successful  polariscopic  observations  were  made  by 
Turner,  Newall,  and  others.  The  inner  corona  showed 
marked  polarization,  a  result  which  was  difficult  to  reconcile 
with  the  absence  of  Fraunhofer  lines  in  the  spectrum  of  the 
corona. 

Interesting  observations  were  made  by  Abbot  with  the 
bolometer  in  measuring  the  radiation  of  the  corona  at  dif- 
ferent distances  from  the  sun's  limb.  Results  of  great  value 
were  secured,  but  they  showed  the  necessity  of  confirming 
them  by  observations  at  succeeding  eclipses. 


CHAPTER  X 

PERSONAL   EXPERIENCES   IN    1900  AND    1901 

first  total  eclipse  witnessed  by  the  author  was 
I  that  of  May  28,  1900.  The  moon's  shadow  path 

JL  in  America  traversed  the  southeastern  states  from 
New  Orleans  to  Norfolk;  it  then  crossed  the  Atlantic  Ocean, 
passed  over  Spain  and  the  Mediterranean  and  left  the  earth's 
surface  at  sunset  in  Northern  Africa.  Good  weather  con- 
ditions had  been  predicted  on  both  sides  of  the  Atlantic  and 
so  it  was  unnecessary  for  the  European  astronomers  to 
travel  to  America  or  for  the  Americans  to  cross  the  Atlantic 
eastward  in  order  to  insure  a  promise  of  successful  opera- 
tions. A  few  astronomers,  however,  did  make  the  sea 
journey.  As  an  illustration  of  how  little  the  average 
European  knows  about  America  —  or  rather  it  should  be 
said,  did  know  of  the  United  States  in  the  year  1900  —  it 
can  be  said  on  good  authority  that  before  crossing  the 
Atlantic  to  witness  the  eclipse,  the  head  of  one  of  the  ex- 
peditions appealed  to  the  United  States  government  for  a 
guard  of  soldiers  to  protect  the  lives  of  the  party  from  the 
wild  natives  (sic)  of  North  Carolina.  On  arriving  in  New 
York,  however,  the  fears  were  effectually  dispelled  when 
the  party  found  themselves  aboard  a  luxurious  Pennsylvania 
railroad  train  and  discovered  that  they  themselves,  their 
baggage  and  their  instruments  were  routed  through  to  their 
destination,  —  and  entirely  free  of  charge. 

In  view  of  the  accessibility  of  the  eclipse  track  in  the 
United  States  and  on  account  of  the  fact  that  it  was  a  very 
favorable  season  of  the  year  for  a  trip  to  the  South,  it  was 
but  natural  that  such  great  American  institutions  as  the 
Lick,  Yerkes  and  Naval  Observatories  should  send  well- 
organized  expeditions,  and  that  individual  astronomers 
should  gather  in  great  numbers  to  witness  the  fascinating 

167 


168  ECLIPSES    OF    THE    SUN 

phenomenon.  Most  of  the  American  astronomers  were 
seeing  their  first  eclipse  and  it  might  well  have  been  thought 
that  they  would  suffer  from  "  nerves "  at  the  exciting 
moments  of  totality;  but  there  were  many  seasoned  veterans 
in  the  ranks,  men  like  Young,  Langley  and  Campbell,  who 
had  witnessed  eclipses  in  foreign  lands. 

Perhaps  the  most  important  part  of  the  program  for  1900 
was  the  spectroscopic  work.  In  1896,  after  many  trials,  the 
first  good  photograph  of  the  flash  spectrum  was  obtained  by 
Shackleton.  In  India  in  1898,  as  a  result  of  better  defini- 
tion, the  spectrum  photographs  exhibited  a  wealth  of  detail 
that  added  greatly  to  our  knowledge  of  solar  physics.  Evi- 
dently the  procedure  for  the  next  eclipse  should  be  the 
attempt  to  secure  spectra  with  increased  dispersion,  so  that 
wave-lengths  should  be  determined  with  greater  accuracy, 
thus  permitting  more  reliable  information  regarding  the 
sources  of  the  spectral  lines  and  rendering  more  positive 
our  knowledge  concerning  motions  of  rotation  of  the  solar 
envelopes.  America  is  the  home  of  the  diffraction  grating, 
plane  and  concave,  brought  to  such  perfection  by  the  refined 
labors  of  Professor  Henry  A.  Rowland  of  Johns  Hopkins. 
Among  the  more  powerful  instruments  brought  into  service 
were  three  concave  gratings,  each  used  with  slit,  and  two 
plane  gratings  each  used  objectively  without  slits.  Each  of 
these  five  gratings  were  employed  in  connection  with  a 
quartz  lens.  Ordinary  photographic  plates  were  used,  and 
as  these  are  but  little  sensitive  to  green,  yellow  and  red 
light,  it  was  decided  to  concentrate  on  the  blue  and  violet 
parts  of  the  spectrum;  hence  the  employment  of  quartz, 
rather  than  glass,  lenses.  The  concave  gratings  were 
mounted  in  the  ordinary  Rowland  manner  in  the  attempt 
to  secure  sharp  definition  and  large  dispersion  —  but  the 
dispersion  was  too  great  for  the  light  available  and  no  lines 
were  found  on  the  photographs.  Huff  obtained  well-exposed 
plates  with  a  plane  grating,  but  the  focus  was  good  for  a 
short  region  only  near  wave-length  4000  A. 

The  experience  gained  in  1900  was  put  to  use  in  the  plans 
for  the  eclipse  of  the  following  year.  On  May  18,  1901,  the 
moon's  shadow  touched  the  earth's  surface  at  sunrise  on  the 


THE  PASIG  RIVER  AT  MANILA 


KRAKATOA  BY  ITS  ERUPTION  IN  1883  CAUSED  A  Loss  OF  40,000  LIVES 


A  COG-RAILROAD  AND  TREE  FERNS  IN  SUMATRA 


THE  WATER  BUFFALO  TAKES  His  DAILY  SIESTA 


PERSONAL    EXPERIENCES    IN    1900    AND    1901      169 

east  coast  of  South  Africa.  After  passing  over  the  Island  of 
Mauritius,  the  shadow  traversed  the  Indian  Ocean  at  cannon- 
ball  speed  and  touched  land  shortly  after  noon  on  the  west 
coast  of  the  Island  of  Sumatra  in  the  Dutch  East  Indies. 
After  crossing  over  northern  Borneo,  Celebes  and  New 
Guinea,  the  shadow  left  the  earth's  surface  at  sunset  far 
south  of  the  Philippines.  This  eclipse  was  specially  im- 
portant on  account  of  the  very  long  duration  of  totality, 
six  minutes.  The  location  that  afforded  the  best  living  con- 
ditions and  promised  the  greatest  success  from  favorable 
weather  was  the  west  coast  of  Sumatra,  but  such  a  site 
would  carry  the  American  astronomers  as  far  away  from 
home  as  they  could  possibly  get  —  half  way  round  the  world. 

In  view  of  this  great  duration  of  totality  and  the  im- 
portance of  the  observations  to  be  attempted,  the  United 
States  government,  through  Congress,  appropriated  money 
to  equip  and  send  out  an  expedition  of  thirteen  members, 
two  from  the  Smithsonian  Institution,  six  from  the  U.  S. 
Naval  Observatory,  and  five  guests  of  the  latter,  all  of  whom 
had  had  eclipse  experience  in  1900,  the  author  being  one  of 
the  five  guests.  In  order  to  arrive  in  the  East  Indies  in 
plenty  of  time  before  the  day  of  the  eclipse,  May  18,  the 
expedition  left  San  Francisco  on  February  16  on  board  the 
U.  S.  Army  Transport  Sheridan,  en  route  to  Manila.  On 
the  journey  westward  three  delightful  days  were  spent  in 
Honolulu,  viewing  the  points  of  interest  of  that  far-off 
"  Paradise  of  the  Pacific. "  At  that  time  the  Hawaiian 
Islands  were  cut  off  from  communication  with  the  outside 
world,  except  in  so  far  as  news  was  brought  by  steamer,  for 
there  was  no  cable  to  Honolulu  —  and  it  was  before  the 
days  of  wireless. 

On  March  26th,  the  U.  S.  S.  General  Alava  left  Manila 
to  carry  the  expedition  the  remaining  2200  miles  to  the 
west  coast  of  Sumatra  where  it  was  intended  to  locate  for 
the  purpose  of  the  eclipse  observations.  After  coasting 
along  the  Philippines  and  the  north  shore  of  Borneo,  the 
equator  was  crossed  on  March  3ist.  Father  Neptune  came 
on  board,  and  the  reception  to  him  was  right  royally  given 
by  the  man-o'-war's  men.  April  2nd  found  us  in  the  straits 


170  ECLIPSES    OF    THE    SUN 

of  Sunda  which  separate  Java  from  Sumatra,  and  the  ship 
passed  within  half  a  mile  of  the  island  volcano  Krakatoa 
whose  eruption  in  1883  resulted  in  the  loss  of  40,000  lives. 
The  huge  tidal  wave  following  the  explosion,  which  inundated 
the  whole  surrounding  country,  left  a  Dutch  man-of-war  high 
and  dry  a  mile  and  a  quarter  inland,  and  76  feet  above  high 
water  mark ;  and  this  wave  was  felt  even  in  the  English  Chan- 
nel, 11,000  miles  away.  The  air  wave  sent  out  by  the  erup- 
tion was  traced  by  barometers  through  seven  complete  cir- 
cuits of  the  earth.  Fine  particles  of  dust  were  shot  up  to 
enormous  distances  in  the  atmosphere,  causing  the  brilliant 
sunsets  noticed  for  many  months  in  1883. 

After  coasting  along  the  west  of  Sumatra,  we  entered  the 
pretty  little  land-locked  harbor  of  Emma  Haven,  probably 
the  first  American  government  ship  that  had  ever  entered 
port  there,  and  were  then  at  the  end  of  our  sea  voyage  of 
over  1 0,000  miles. 

After  official  calls  on  the  governor  of  that  peaceful  Dutch 
colony,  we  proceeded  to  Padang,  the  capital,  just  four  miles 
distant,  and  obtained  our  first  view  of  the  island  which  was 
to  be  our  home  for  the  next  two  months.  First  impressions 
were  formed  at  the  Oranje  Hotel,  a  typical  hostelry  of  the 
East.  The  building,  half  hidden  by  cocoanut  and  tropical 
palms,  is  of  one  story  with  high  thatched  roof.  At  the  front  is 
the  wide  open  verandah  —  the  parlor  and  reception  room 
of  the  hotel  —  and  back  of  this,  running  through  the  middle 
of  the  building,  is  the  dining  room.  On  either  side  are  the 
spacious  bed  rooms,  each  with  its  wide  verandah  —  exceed- 
ingly inviting  places  in  the  hot  afternoons.  The  bed  is  the 
chief  curiosity  to  the  American,  for  it  is  a  four-poster,  with 
finely-figured  mosquito  net  and  of  the  huge  dimensions  of 
7x8  feet.  Over  the  mattress  is  spread  a  daintily  embroid- 
ered sheet,  but  there  are  no  bed-covers  —  and  indeed,  under 
the  equator,  few  blankets  are  needed.  Of  pillows,  there  are 
several,  the  ordinary  bed  having  four  and  two  "  Dutch  wives  " 
—  the  name  given  to  a  long  bolster  used  to  keep  the  occupant 
of  the  bed  cool. 

A  day  in  the  East  is  begun  by  awaking  about  six  o'clock. 
Coffee  is  immediately  brought  by  the  jungus  —  for  no  white 


PERSONAL    EXPERIENCES    IN    1900    AND    1901      171 

man  does  any  manual  labor,  and  each  has  his  native  valet 
—  and  then,  clad  in  pajamas,  one  risks  the  bath.  Such  a 
thing  as  a  bath  tub  is  unknown,  the  bath  being  taken  in  a 
cemented  room  with  a  cistern  of  water  at  the  back,  over 
the  top  of  which  is  placed  a  wire  screen  to  keep  out  inquisi- 
tive foreigners.  Dipping  water  through  a  rectangular  open- 
ing in  the  screen  by  means  of  a  bucket,  and  throwing  it  over 
oneself,  constitutes  the  bath  —  and  a  very  excellent  one  it 
proved  to  be.  After  leisurely  dressing  (for  no  one  is  ever 
in  a  hurry),  a  light  breakfast  is  partaken  of,  consisting  usu- 
ally of  bread  and  a  few  cold  meats.  The  men  come  to  break- 
fast dressed  for  the  business  of  the  morning,  in  their  white 
suits,  or  often  in  their  pajamas,  consisting  of  trousers  of 
cotton  stuff  of  the  most  marvelous  colors  and  patterns,  and 
a  coat  or  kibaya  of  white,  made  after  the  Chinese  style 
without  collar.  But  how  tell  of  the  dress  of  the  ladies?  For 
they  appear  at  breakfast  in  costumes  in  which  an  American 
girl  might  be  ashamed  to  be  caught  in  her  boudoir:  a  native 
skirt  or  sarong  reaching  to  the  ankles  made  of  picturesque 
cotton  stuff,  and  a  lace-edged  linen  jacket.  This  is  the  dress 
of  the  native  women  who  wear  neither  shoes  nor  stockings. 
The  Dutch  women  have  adopted  their  costume  in  toto,  except 
that  they  usually  add  a  pair  of  gold  embroidered  slippers. 
After  breakfast,  the  serious  business  of  the  day  is  taken  up 
until  it  is  time  for  the  midday  meal,  or  rijsttajel.  This,  as  the 
name  signifies,  is  largely  made  up  of  rice  —  and  it  is  a  most 
astonishing  meal.  Into  a  large  soup  plate  is  put  a  liberal 
supply  of  splendidly  cooked  rice,  and  on  top  of  this,  chicken, 
meat,  potatoes,  and  portions  of  fifteen  or  twenty  curried 
dishes.  It  makes  an  astounding  looking  mess,  but  after  a 
little  experience  in  selecting  the  proper  proportions  and 
combinations  of  the  curry  and  spices,  the  whole  makes  a  very 
palatable  dish.  This  is  only  the  first  course,  to  be  followed 
by  potatoes  fried  in  cocoanut  oil,  and  excellent  chicken,  with 
a  third  course  of  fruit.  The  next  two  hours  of  the  day  are 
universally  given  up  to  the  siesta.  About  four  o'clock,  the 
East  again  awakes,  and  after  a  cup  of  tea  or  chocolate,  takes 
another  bath  and  dresses  for  the  afternoon.  The  Dutchman 
seeks  his  club  and  plays  a  game  of  billiards  or  cards.  Games 


172  ECLIPSES    OF    THE    SUN 

requiring  much  exercise  do  not  find  favor  in  his  eyes,  but  he 
has  a  taste  for  horse  flesh,  breeds  most  wonderful  little  ponies, 
and  is  always  riding  or  driving.  The  ladies  now  appear  for 
the  first  time  during  the  day  in  European  costyme ;  and  the 
hours  before  late  dinner  are  given  up  to  social  functions.  If 
these  are  of  a  ceremonious  nature,  the  men  must  wear  a  black 
tail  coat,  and  at  least  carry  a  hat,  even  if  it  is  not  worn. 

A  nice  little  informal  gathering  takes  place  before  dinner 
each  evening  on  the  hotel  verandah,  where  the  men  drink 
their  pitje  of  gin  and  bitters,  furnished  by  the  hotel  to  all 
comers.  Dinner  at  8:30  or  9:00  has  nothing  remarkable 
about  it,  being  very  similar  to  our  own.  As  soon  as  dinner 
is  over,  negligee  is  resumed,  and  the  day  is  at  an  end. 

Within  a  week  there  arrived  in  Padang  two  other  parties 
besides  our  own  from  the  United  States,  and  scientists  from 
England,  Holland,  France,  Russia,  Japan,  and  even  India, 
about  eighty  astronomers  altogether  from  all  parts  of  the 
world,  all  bent  on  learning  something  about  the  sun. 

The  Dutch  were  very  kind  and  generous  in  their  treat- 
ment of  the  foreign  scientists,  and  did  everything  in  their 
power  to  make  the  two  months'  stay  in  their  island  as  easy 
in  becoming  acquainted  with  strange  conditions  and  as  en- 
joyable, as  possible.  Passes  were  furnished  to  each  astrono- 
mer for  passage  over  the  "  Staatsspoorweg  op  Sumatra/'  the 
railroad  owned  by  the  government  which  runs  from  Padang 
into  the  interior  about  100  miles,  one  spur  reaching  Fort  de 
Kock,  the  other  Sawah  Loento.  These  passes  included  not 
only  the  transportation  of  our  persons,  but  also  of  freight 
and  baggage,  which  is  not  carried  free  in  the  island.  Labor 
was  largely  furnished  without  cost,  machine  shops  were  put 
at  the  disposal  of  the  astronomers,  and  their  slightest  wish 
was  readily  met  and  generally  anticipated.  In  short,  these 
Hollanders  were  perfect  models  of  hospitality. 

In  a  few  days  after  arrival,  the  astronomers  had  separated 
to  the  different  locations  determined  on  as  bases  of  opera- 
tions. The  English  divided  into  two  parties,  one  going  to 
Sawah  Loento,  where  were  located  Mr.  and  Mrs.  Newall, 
the  other  to  an  island  off  the  coast  of  Sumatra  where  the 
party  under  Dyson  could  have  the  assistance  of  the  officers 


PERSONAL    EXPERIENCES    IN    1900    AND    1901      173 

and  men  of  H.  M.  S.  Pigmy,  detailed  there  to  help  in  the 
observations.  The  Dutch  located  not  far  from  this  island 
but  on  the  mainland,  and  the  astronomers  had  the  help  of 
the  gunboat  Sumatra.  Perrine  of  the  Lick  Observatory  of 
California  decided  on  Padang  as  likely  to  be  most  favorable 
for  his  researches. 

The  Naval  Observatory  divided  their  party  into  three  sec- 
tions, and  decided  to  locate  along  the  line  of  the  railroad. 
The  engineering  principles  under  which  this  road  was  con- 
structed were  marvels  of  simplicity.  If  a  small  hill  was  met 
in  laying  out  the  line,  the  rails  were  laid  over  it,  if  the  hill 
proved  too  steep  for  this,  the  tracks  went  around  it.  As  the 
road  starts  from  sea  level  and  reaches  an  elevation  of  4,000 
feet  in  less  than  100  miles,  following  a  valley  between  moun- 
tains of  9,000  feet,  there  are  many  steep  grades  and  sharp 
curves;  in  fact,  for  half  the  distance  it  becomes  a  cog  road. 
But  it  is  one  of  the  most  picturesque  routes  imaginable, 
running  now  through  the  green  rice  fields,  now  past  fields 
ready  for  the  sickle  —  for  there  are  no  seasons,  and  sowing, 
reaping,  and  threshing  may  be  under  way  in  the  same  field 

—  now  through  a  grove  of  cocoanut  and  banana  palms,  and 
again  through  a  dense  tropical  jungle,  where  a  dozen  chatter- 
ing monkeys  scared  up  by  the  train  go  swinging  away  from 
limb  to  limb.    On  our  first  trip  over  the  road  we  had  just 
come  to  a  bridge  and  were  going  down  a  steep  grade  when 
the  Malay  brakeman  in  some  way  lost  his  headgear.    While 
we  sat  there  for  half  a  second  wondering  what  he  would  do 

—  for  we  had  never  seen  a  native  without  his  topi  —  he 
solved  the  difficulty  by  jumping  off  the  train,  running  back 
after  his  cap,  and  after  a  short  run  of  50  yards,  catching  the 
train  again,  which  all  this  time  was  moving  at  its  usual 
speed.    At  one  of  the  stations  where  we  got  off  for  a  few 
minutes  to  look  around,  we  were  immediately  the  center  of 
a  large  crowd  of  most  curious  natives.     They  offered  us 
bananas  —  pisang  as  they  call  them  —  and  oranges,  mostly 
green  in  color,  little  pieces  of  sugar-cane  on  slivers  of  wood, 
and  many  strange  and  wonderful  looking  messes.     They 
eyed  us  with  much  wonder,  and  attempted  a  few  words  in 
Malay,  which  we  however  understood  about  as  well  as  they 


174  ECLIPSES    OF    THE    SUN 

did  our  English.  In  a  few  days  however,  they  became  ac- 
quainted with  our  mission,  and  spoke  of  us  by  the  Dutch  word 
"Zoneclips,"  while  we,  on  the  other  hand,  learned  to  make 
ourselves  understood  in  Malay. 

It  was  necessary  to  select  the  three  Naval  Observatory 
eclipse  stations  on  the  line  of  the  railroad,  on  account  of 
the  difficulty  of  transporting  heavy  instruments  and  supplies 
into  the  interior.  At  Fort  de  Kock,  near  the  edge  of  the 
moon's  shadow  path,  three  members  under  the  direction  of 
Professor  W.  S.  Eichelberger,  U.  S.  N.,  investigated  the 
corona  and  the  atmosphere  of  the  sun  with  photographic 
telescope  and  spectroscope.  At  Solok,  the  main  station  of 
the  party  was  located  under  the  direction  of  Professor  A.  N. 
Skinner,  U.  S.  N.  Here  was  Professor  Barnard  with  a  pho- 
tographic telescope  of  6i^-foot  focus,  which  would  give  an 
image  of  the  sun  about  seven  inches  in  diameter.  Barnard 
planned  to  give  several  short  exposures,  together  with  a  single 
exposure  of  two  and  a  half  minutes,  on  a  plate  40  inches 
square,  hoping  thereby  to  obtain  some  exquisite  details  of  the 
corona.  At  Solok  were  also  located  several  spectroscopes 
under  the  supervision  of  Jewell  who  was  engaged  in  various 
interesting  objects  of  research. 

Abbot  of  the  Smithsonian  Institution  was  investigating 
along  two  separate  lines;  first,  with  four  telescopes  mounted 
so  as  to  photograph  a  large  region  in  duplicate  in  the  vicinity 
of  the  sun,  he  was  attempting  to  discover  new  members  of 
the  solar  system  revolving  inside  of  the  orbit  of  the  planet 
Mercury  (if  there  be  any  such  objects).  His  second  task 
was  a  plan  for  measuring  the  heat  of  the  corona  and  of  the 
dark  side  of  the  moon  turned  towards  us  with  the  delicate 
bolometer,  which  is  in  reality  nothing  but  a  very  sensitive 
thermometer  capable  of  detecting  the  heat  of  an  ordinary 
candle  at  the  distance  of  five  miles  and  of  measuring  dif- 
ferences of  temperature  to  the  one-millionth  of  a  degree. 

The  author  was  in  charge  of  the  U.  S.  Naval  Observatory 
party  at  Sawah  Loento  at  the  terminus  of  the  government 
railroad.  The  sawah,  or  irrigated  rice-fields,  is  one  of  the 
prettiest  sights  imaginable.  These  fields  are  usually  on  the 
side  of  a  hill,  for  the  west  coast  of  Sumatra,  where  the  as- 


PERSONAL    EXPERIENCES    IN    1900    AND    1901      175 

tronomers  were  located,  is  extremely  rough  and  rugged. 
The  sides  of  the  hills  have  been  terraced  in  order  that  the 
water  may  flow  from  one  level  to  the  one  next  lower,  the 
enormous  amount  of  manual  labor  required  to  make  these 
terraces  being  for  the  most  part  performed  by  the  women. 
The  rice  grows  under  water,  but  artificial  irrigation  is  not 
needed,  for  it  rains  almost  every  day,  not  merely  a  shower, 
but  with  an  average  fall  of  half  an  inch  for  every  day  of  the 
year.  A  view  of  these  rice  terraces  with  their  vivid  tropical 
greens  —  and  such  brilliant  color  is  never  seen  in  the  tem- 
perate zones  —  is  certainly  a  magnificent  sight.  All  trace  of 
the  sawah,  however,  has  departed  from  the  village  of  Sawah 
Loento,  and  its  place  has  been  taken  by  the  Ombilian  Coal 
mines,  belonging  to  and  operated  by  the  Dutch  government, 
which  are  remarkable  for  having  a  vein  of  rich  coal  forty 
feet  in  thickness. 

The  elevation  of  the  railway  station  at  Sawah  Loento, 
according  to  the  Dutch  maps,  is  262  meters  above  sea  level, 
or  859  feet.  The  height  of  the  eclipse  station  above  this 
was  measured  by  means  of  an  aneroid  barometer  as  400  feet, 
which  therefore  made  the  elevation  of  the  eclipse  camp 
about  1,260  feet  above  sea  level.  A  short  distance  away 
was  the  expedition  from  the  Massachusetts  Institute  of 
Technology  consisting  of  Burton,  Hosmer,  Harrison  Smith 
and  Matthes. 

The  solar  problems  to  be  investigated  by  the  Naval  Ob- 
servatory party  at  Sawah  Loento  were  along  two  separate 
lines:  to  photograph  the  corona  with  a  camera  104  inches 
long,  with  lens  six  inches  in  diameter,  and  secondly,  to 
photograph  the  spectrum  of  the  chromosphere  with  a  plane 
grating  and  quartz  lens,  used  objectively  without  slit.  To 
mount  the  instruments,  piers  of  brick  were  constructed. 
Tents  were  used  as  coverings  for  the  instruments,  with  an 
extra  one  for  a  storehouse,  four  tents  altogether  being  set 
up.  As  the  Boston  party  had  reached  Sawah  Loento  about 
ten  days  before  us,  they  had  learned  to  some  extent  the  best 
way  in  which  to  proceed.  The  benefit  of  their  experience 
was  freely  imparted,  and  much  valuable  information  cheer- 
fully given.  To  Meinheer  van  Lessen,  the  chief  engineer 


176  ECLIPSES    OF   THE    SUN 

of  the  coal  mines,  many  thanks  were  due  for  the  very  gen- 
erous way  in  which  he  looked  after  the  supplying  of  all 
building  material.  The  bricks,  made  near  the  coal  mines, 
were  transported  to  the  eclipse  location  in  three  stages ;  first 
by  rail  to  the  residence  of  the  controller;  second,  by  the 
slow-moving  "kreta  kerbau,"  drawn  by  the  sturdy  water- 
buffalo;  and  third,  for  the  remainder  of  the  distance,  about 
a  third  of  a  mile,  by  coolies.  The  slowness  of  the  last  part 
of  the  operation  can  perhaps  be  imagined  when  it  is  stated 
that  in  a  basket  slung  on  a  bamboo  pole  on  the  shoulders 
of  two  coolies,  five  ordinary  sized  bricks  would  be  carried. 
This  was  indeed  the  minimum  load,  but  the  maximum  was 
never  more  than  ten.  It  was  very  interesting  to  see  six  coolies, 
using  three  bamboo  poles,  carrying  a  barrel  of  cement.  The 
coolies  provided  were  convicts.  The  way  in  which  the  Dutch 
East  Indies  treat  their  prisoners  is  one  of  the  interesting  fea- 
tures in  the  management  of  the  Malay.  If  a  native  of  Su- 
matra commits  a  crime,  he  is  sent  to  one  of  the  other  East 
India  islands ;  and  similarly  natives  of  the  other  islands  are 
sent  to  Sumatra  to  serve  out  the  terms  of  their  punishment. 
Consequently,  in  a  penal  settlement  in  Sumatra  are  natives  of 
Java,  Borneo,  Celebes  and  of  some  of  the  smaller  islands,  but 
no  Sumatrans.  The  reason  for  this  strange  separation  is  that 
there  is  great  enmity  between  the  different  races,  and  so  if  a 
Javanese  prisoner  tries  to  escape  he  is  immediately  appre- 
hended by  the  first  Sumatran  who  meets  him  and  sent  back. 
And  thus  it  is  that  there  are  large  penal  settlements  with  no 
surrounding  walls  and  very  few  guards. 

Such  a  settlement  was  at  Sawah  Loento.  The  convicts,  to 
the  number  of  3 ,000,  were  employed  in  the  coal  mines.  There 
was  no  guard  over  them  except  the  "  mandur  "  or  policeman, 
one  of  themselves  raised  to  a  position  of  authority  and 
responsibility,  and  answerable  for  the  conduct  of  the 
coolies  under  him.  Notwithstanding  these  conditions,  one 
could  go  around  with  perfect  safety  and  with  less  fear  of 
molestation  than  in  New  York  City.  In  fact,  the  pistols 
we  had  brought,  with  which  to  defend  ourselves  from  canni- 
bals, were  soon  packed  carefully  away  in  the  bottom  of  our 
trunks. 


A  SUMATRAN  BEAUTY 


MARKET  SCENE  IN  SUMATRA 


SUMATRAN  MOTHER  AND  HALF-CASTE  DAUGHTERS 


EAST  INDIANS  IN  PICTURESQUE  GARBS 


PERSONAL    EXPERIENCES    IN    1900    AND    1901      177 

In  order  to  make  the  convicts  more  satisfied  with  their  lot, 
the  Dutch  authorities  paid  them  the  amount  of  seven  cents 
Dutch  money,  2.8  cents  American,  per  day.  There  seemed 
to  be  a  great  amount  of  sickness  among  the  convicts,  pos- 
sibly due  to  their  confinement  in  the  hot  country,  and  several 
hundred  of  them  were  in  hospitals  or  were  put  on  sick  duty 
without  pay.  These  coolies  were  provided  for  us  by  the  con- 
troller free  of  charge,  and  we  were  able  to  have  about  as 
many  as  we  needed.  They  were  very  slow  and  not  over  fond 
of  work,  but  nevertheless  they  could  carry  bricks  about  as  fast 
as  our  Malay  "  tukang  "  could  lay  them.  He  used  to  squat 
at  his  work,  and  it  took  five  days  for  him  to  lay  2,200  bricks. 
But  the  piers  were  finally  built,  the  tents  raised,  and  the  in- 
struments gradually  mounted. 

The  day  of  the  eclipse  dawned  clear,  and  our  hopes  were 
that  these  favorable  conditions  would  remain  until  after 
totality,  which  occurred  shortly  after  noon.  First  contact 
was  observed  in  a  perfectly  clear  sky,  but  soon  after  this, 
clouds  began  to  gather,  and  a  quarter  of  an  hour  before  second 
contact  the  sky  was  completely  overcast. 

The  disappearing  crescent  of  the  sun  was  observed  with 
a  binocular  before  one  barrel  of  which  was  arranged  a  small 
plane  grating  in  such  a  way  that  with  one  eye  the  spectrum 
could  be  seen,  and  with  the  other  eye  the  sun  itself.  With 
this,  shortly  before  the  time  of  second  contact,  bright  lines 
were  seen  for  a  few  seconds  at  F  and  H  and  in  several  places 
in  the  green  and  yellow,  but  these  disappeared  almost  at 
the  instant  of  being  seen,  the  sun  being  completely  hidden 
by  clouds,  and  the  flash  passed  without  our  being  able  to  see  it. 

Toward  the  middle  of  totality,  conditions  became  a  trifle 
better,  so  that  it  was  possible  to  see,  through  clouds,  the 
corona  extending  for  about  half  a  diameter  from  the  sun, 
and  with  the  small  spectroscope  to  trace  the  form  of  the 
coronium  line  quite  distinctly.  During  no  time  of  the  Sm  41" 
of  totality  was  an  unclouded  view  of  the  corona  obtained, 
but  nevertheless,  the  clouds  were  so  thin  at  the  end  of  totality 
that  the  second  flash  was  beautifully  seen. 

One  hour  after  the  total  phase  the  clouds  cleared  away 
and  a  perfect  sky  remained  for  the  rest  of  the  day.  Alas! 


178  ECLIPSES    OP    THE    SUN 

that  the  eclipse  did  not  occur  at  one  o'clock  instead  of 
twelve ! 

The  pre-arranged  program  was  carried  out,  however,  in  its 
entirety  as  if  it  had  been  clear. 

Powerful  spectrographs  were  used  also  at  the  two  other 
Naval  Observatory  stations.  At  the  main  station  near  Solok, 
Jewell  had  a  concave  grating,  but  as  the  result  of  the  ex- 
perience in  1900,  the  instrument  was  used  without  a  slit. 
Dinwiddie  had  a  prismatic  camera  under  his  care,  a  single 
large  prism  of  60°  angle,  kindly  loaned  by  the  Smithsonian 
Institution.  Near  the  edge  of  the  moon's  shadow-path  at 
Fort  de  Kock,  Humphreys  had  a  huge  concave  grating  of 
nine  inches  aperture,  with  lines  three  inches  in  length  and  a 
radius  of  curvature  of  thirty  feet.  This  was  the  largest 
grating  ever  ruled  on  Rowland's  dividing  engine  and  it  was 
constructed  specially  for  the  purpose  of  this  eclipse.  It  was 
not  a  perfect  specimen  of  a  grating  for  the  diamond  point 
had  broken  down  during  the  course  of  the  ruling.  This 
station  was  purposely  located  near  the  edge  of  the  shadow 
in  order  to  photograph  the  low-lying  layers  of  the  chromo- 
sphere. Weather  statistics  did  not  promise  favorable  skies 
at  Fort  de  Kock  —  but  strange  to  relate,  it  was  the  only 
locality  in  Sumatra  where  astronomers  were  stationed  that 
clouds  were  not  experienced.  The  main  station  at  Solok 
was  far  from  being  fortunate,  and  the  astronomers  there  suc- 
ceeded in  getting  —  almost  nothing,  so  dense  were  the  clouds 
throughout  the  whole  of  totality. 

From  Jewell's  many  years  at  Johns  Hopkins  University, 
working  as  assistant  to  Rowland,  he  had  had  vast  experience 
in  the  development  of  spectroscopic  plates,  and  hence  the 
task  of  developing  the  precious  negatives  secured  was  handed 
over  to  him.  While  this  was  being  done,  and  while  the  instru- 
ments were  being  dismounted  and  boxed  and  crated  for  their 
long  journey  home,  we  had  some  days  to  look  around  and 
observe  the  manners  and  customs  of  the  people.  The  natives 
are  all  of  them  frightfully  lazy  —  or  shall  we  say  only  tired? 
But  it  is  the  same  disease  that  afflicts  the  natives  all  through 
the  East.  The  Filipinos  are  troubled  in  the  same  manner,  a 
similarity  not  to  be  wondered  at,  for  Sumatrans  and  Filipinos 


PERSONAL    EXPERIENCES    IN    1900    AND    1901      179 

both  belong  to  the  same  great  parent  family.  It  was  prob- 
ably for  this  reason  that  the  natives  of  the  Dutch  East  Indies 
were  so  interesting. 

The  Malay  is  naturally  a  man  of  easy-going,  indolent 
character,  who  never  gives  open  expression  to  a  sense  of  as- 
tonishment or  fear,  and  is  probably  little  affected  by  these 
feelings.  When  alone,  he  is  gloomy  and  taciturn,  never  either 
singing  or  talking  to  himself.  The  upper  classes  are  exceed- 
ingly courteous,  yet  this  outward  refinement,  strange  to  say, 
co-exists  with  the  most  pitiless  cruelty  and  contempt  of  hu- 
man life,  traits  which  belong  to  the  dark  side  of  their  char- 
acter ;  and  herein  lies  the  explanation  of  the  many  diametri- 
cally opposed  judgments  which  have  been  given  us  of  the 
native  of  the  East.  There  is  another  trait  we  must  not  for- 
get, and  that  is  an  insatiable  love  of  gambling  which  no  laws 
seem  to  be  able  to  suppress. 

These  are  the  characteristics  of  the  Malay,  but  they  de- 
scribe the  Filipino  just  as  accurately  as  they  do  the  native 
of  Sumatra.  We  were  almost  two  months  in  Sumatra  and 
in  that  time  were  thrown  constantly  with  the  natives  who, 
as  servants  and  coolies,  were  used  continually.  It  became 
necessary  to  learn  Malay  and  speak  it ;  this  however  proved 
not  to  be  a  very  difficult  task  as  the  language  contains  neither 
declensions  nor  conjugations. 

The  horned-roofed  houses  were  very  picturesque,  and  near 
Fort  de  Kock  one  of  these  was  seen  being  modelled  in  gold 
and  silver  filigree  work  (at  which  the  natives  are  very  skill- 
ful), for  a  present  from  the  colony  to  the  Queen  of  Holland, 
to  whom  they  are  very  loyal.  The  natives  have  a  pretty 
legend,  the  Menangkarbau,  about  the  origin  of  the  horn- 
shaped  roofs.  This  relates  that  there  was  once  eternal 
enmity  between  the  Javanese  and  Sumatrans,  first  one  side 
conquering  and  then  the  other,  and  it  seemed  likely  that  the 
bloody  conflict  would  continue  without  anything  ever  being 
decided.  Finally  the  sages  of  the  two  peoples  hit  upon  a 
plan  to  the  effect  that  each  nation  should  choose  a  champion 
from  the  animal  world,  that  these  should  meet  in  mortal 
combat,  and  that  whichever  animal  was  victor,  that  nation 
was  to  be  considered  the  conqueror  for  all  time.  The  Java- 


180  ECLIPSES    OF    THE    SUN 

nese  chose  a  vigorous  young  tiger,  under  the  impression  that 
their  champion  surely  could  not  be  beaten,  while  the  fore- 
fathers of  the  Sumatran  natives  picked  a  strong  two-year- 
old  karibou  bull.  Fancy,  if  we  can,  the  two  tribes  in  half 
circles  meeting  to  see  the  fray,  the  shouts  of  rejoicing  on  one 
side  and  the  woes  of  the  other  when  the  karibou  succeeds  in 
using  its  horns  to  such  advantage  that  the  tiger  is  killed.  To 
perpetuate  this  victory,  the  Sumatrans  forever  after  build 
their  roofs  to  show  the  karibou  horns.  Usually  the  roof  has 
one  pair  of  horns,  but  it  may  have  two  or  three  pairs,  the  extra 
pairs,  we  are  told,  signifying  that  a  daughter  is  married  in  the 
house.  For  the  family  in  Sumatra  is  maternal,  with  the 
mother  as  head  of  the  house,  so  when  a  daughter  is  married 
she  brings  her  husband  to  live  in  her  mother's  home.  Strange 
to  relate,  this  form  of  family  government  coexists  with  Mo- 
hammedanism, where  a  man  may  have  three  or  four  wives. 
In  such  cases,  however,  we  were  never  given  to  understand 
just  how  a  man  divided  his  time,  and  how  he  could  live  peace- 
ably with  so  many  mothers-in-law.  Women  do  most  of  the 
work,  till  the  soil,  gather  the  crops,  thresh  the  rice  and  carry 
all  the  burdens,  while  the  man  evidently  considers  himself  a 
superior  being  and  succeeds  in  doing  very  little  work.  No 
wonder  then  he  has  three  or  four  wives  to  work  for  him ! 

The  Dutch  have  kept  the  original  tribal  relation  and  each 
tribe  still  has  its  chief,  who  is  however,  little  more  than  a 
figurehead.  Each  man  is  required  to  perform  one  hundred 
days  labor  each  year  for  the  government,  and  as  a  great 
amount  of  this  is  put  on  the  roads,  excellent  carriage  roads 
appear  even  in  almost  inaccessible  parts  of  the  island.  The 
principle  under  which  the  Dutch  run  their  colonies  has,  by 
some  writers,  been  described  as  merely  for  the  purpose  of 
making  money  out  of  them,  and  hence  it  has  been  the  policy 
to  keep  the  natives  from  becoming  educated  or  Christianized, 
to  keep  out  European  immigrants  and  capitalists  and  to  pre- 
serve the  whole  trade  as  a  monopoly  of  the  home  government. 
And  so  we  hear  of  the  "  culture  system/'  but  the  products 
under  the  "  system  "  have  gradually  been  reduced,  until  now 
in  Sumatra  the  chief  ones  are  coffee  and  tobacco  —  and  who 
has  not  heard  of  "  Sumatra  wrappers  "?  Natives  are  obliged 


PERSONAL    EXPERIENCES    IN    1900    AND    1901      181 

to  plant  a  certain  number  of  coffee  trees  each  year,  and  sell 
the  product  at  a  fixed  rate  to  the  government.  The  Dutch 
have,  however,  been  exceedingly  successful  in  keeping  the 
Malays  docile  and  contented,  and  it  might  be  well  for  the 
Americans  to  study  the  results  of  these  three  hundred  years 
experiment  on  Malays  of  exactly  the  same  characteristics  as 
the  Malays  in  the  Philippines,  the  Filipinos. 

After  traveling  half  way  round  the  world  in  search  of 
knowledge,  to  have  cloudy  weather  during  the  precious  six 
minutes  of  the  eclipse  was  indeed  heart-rending,  and  the 
majority  of  the  astronomers  were  pretty  blue  as  a  result. 
But  when  the  plates  came  to  be  developed  —  and  what  a 
boon  the  photographic  plate  has  been  to  the  astronomer 
—  it  was  found  that  the  clouds  had  not  interfered  quite  as 
much  as  expected.  And  so,  down  on  the  coast,  Dyson  and 
the  Dutch  parties,  with  the  assistance  of  their  warships,  suc- 
ceeded in  getting,  with  telescope  and  spectroscope,  some 
really  excellent  photographs  of  the  corona  and  "  flash."  The 
corona  could  not  be  traced  a  very  great  distance  from  the 
sun,  but  many  exquisite  details  were  seen  in  the  inner  corona. 
At  Padang,  Perrine  of  the  Lick  Observatory  had  the  same 
kind  of  cloudy  weather  as  experienced  by  nearly  all  the 
other  scientists,  yet  the  results  of  his  work  were  very  satis- 
factory in  spite  of  the  clouds.  The  photographs  of  the  corona 
taken  with  the  4O-foot  photoheliograph,  like  those  of  the 
English  and  Dutch  parties,  showed  splendid  detail  in  the  inner 
corona,  but  they  were  particularly  interesting  from  an  ap- 
pearance in  the  northeasterly  portion  of  the  corona,  as  if  an 
explosion  had  taken  place.  Perrine  later  found  that  this 
remarkable  disturbance  was  immediately  above  the  promi- 
nent and  only  sun-spot  visible  during  eleven  days,  thus  show- 
ing an  intimate  connection  between  sun-spots  and  disturb- 
ances in  the  sun. 

The  three  parties  at  Sawah  Loento  suffered  likewise  from 
the  clouds.  The  main  work  of  Burton  and  his  party  from 
the  Massachusetts  Institute  of  Technology  was  investigat- 
ing the  magnetic  disturbances  while  the  eclipse  was  in  prog- 
ress; this  work,  however,  could  be  carried  out  as  well  in 
cloudy  weather  as  in  clear.  The  Boston  party  found  the 


182  ECLIPSES    OF    THE    SUN 

disturbance  not  so  great  as  at  the  eclipse  of  1900.  Newall 
of  Cambridge,  England,  had  a  very  complete  spectroscopic 
program  to  carry  out,  including  many  interesting  researches, 
among  which  was  an  attempt  to  measure  the  velocity  of  rota- 
tion of  the  corona  and  to  see  whether  or  not  this  halo  shines 
by  its  own  inherent  light. 

The  few  days  previous  to  departure  were  spent  at  Padang 
where  we  had  the  pleasure  of  meeting  many  of  the  visiting 
astronomers  whose  accommodation  taxed  to  the  utmost  the 
capacity  of  the  leading  hotels,  the  Oranje  and  the  Atjeh.  On 
the  evening  of  May  27,  the  United  States  consular  agent  gave 
a  dinner  and  farewell  ball  in  honor  of  the  American  astrono- 
mers and  naval  officers.  Many  prominent  people  were  pres- 
ent, including  Governor  Joekes,  also  the  officers  of  H.  M.  S. 
Pygmy.  After  the  ball  was  over,  the  author  slept  in  one  of  the 
beds  of  the  Oranje  Hotel  with  Professor  Barnard,  Dr.  Abbot 
and  one  other.  Our  ship,  the  U.  S.  S.  General  Alava,  had 
waited  in  Emma  Haven  to  carry  us  back  to  Manila  and  we 
sailed  at  10  A.M.  May  28  on  our  homeward  voyage.  The 
eclipse  of  1901  was  a  thing  of  the  past!  On  June  8  we  en- 
tered Manila  Bay  through  the  north  channel,  the  Boca  Chica, 
and  anchored  off  Cavite.  After  a  fortnight's  wait  in  Manila, 
we  sailed  aboard  the  U.  S.  Army  Transport  Indiana,  and 
reached  San  Francisco  on  July  16,  five  months  from  the  day 
we  had  departed. 

What  scientific  results  had  been  obtained  from  the  long 
journey  and  the  many  months  spent  away  from  regular 
duties?  When  the  photographic  plates  were  studied  in  de- 
tail it  was  found  that  much  of  great  value  had  been  secured 
in  spite  of  the  clouds,  and  as  a  result  the  Sumatra  eclipse 
had  contributed  some  very  important  information  regarding 
the  problems  of  solar  physics. 


CHAPTER  XI 

THE  SPANISH  ECLIPSE  OF  1005 

next  eclipse  to  be  widely  observed  was  that  of 
August  30,  1905.  On  this  day  the  eclipse  began 
in  Manitoba  and,  after  crossing  through  Northern 
Canada,  it  left  Labrador  about  8  A.M.  on  its  trip  across  the 
Atlantic.  Shortly  after  noon  the  shadow  cut  into  Spain, 
then  on  through  the  Mediterranean,  Northern  Africa  and 
Egypt,  leaving  the  earth's  surface  at  sunset  on  the  coast 
of  the  Indian  Ocean. 

Spain  was  chosen  by  the  majority  of  astronomers,  both 
because  the  duration  of  totality  was  longer,  and  because  the 
promise  of  good  weather  conditions  was  better;  and  here  in 
a  path  one  hundred  and  twenty  miles  in  width  running  diago- 
nally across  the  peninsula,  hundreds  of  astronomers,  Ameri- 
can and  European,  were  gathered. 

The  party  sent  out  by  the  United  States  government  was 
under  the  general  direction  of  Rear-Admiral  Colby  M. 
Chester,  U.  S.  N.,  superintendent  of  the  Naval  Observatory. 
Three  men-of-war  were  furnished  by  the  Navy  Department 
for  the  purposes  of  the  expedition,  the  U.  S.  S.  Minneapolis, 
U.  S.  S.  Dixie  and  U.  S.  S.  Caesar,  the  first  named  being  the 
flagship  of  the  squadron. 

The  three  vessels  left  separately  from  the  United  States 
about  the  end  of  June  and  met  in  Gibraltar  about  the  middle 
of  July.  "  Gib  "  is  one  of  the  most  interesting  places  in  the 
world,  especially  when  entering  on  a  naval  vessel.  It  was 
a  glorious  sight,  as  we  steamed  in  at  dawn  on  board  the 
Minneapolis,  to  behold  the  wonderful  rock  and,  sheltered  at 
its  base,  the  Mediterranean  squadron  of  the  British  navy, 
consisting  of  eight  battleships  and  eight  first-class  cruisers, 
under  the  greatest  of  English  admirals  of  the  time,  Lord 

183 


184  ECLIPSES    OF    THE    SUN 

Charles  Beresford.  The  morning  of  our  arrival  was  spent 
in  firing  and  acknowledging  thunderous  salutes,  and  in  mak- 
ing official  calls.  To  carry  out  properly  these  acts  of  courtesy 
between  the  American  and  British  nations,  it  was  necessary 
to  fire  no  less  than  one  hundred  and  fifty-two  rounds  of  am- 
munition. On  the  morning  of  our  second  day  in  Gibraltar, 
the  British  squadron  sailed,  and  it  gave  us  an  idea  of  the 
quality  of  the  greatest  navy  in  the  world  to  see  the  splendid, 
seamanlike  manner  in  which  the  big  ships  got  under  way 
without  confusion,  and  one  by  one  in  perfect  order  departed 
from  the  crowded  harbor. 

After  leaving  Gibraltar  and  entering  the  blue  waters  of 
the  Mediterranean,  the  Minneapolis  steamed  along  the  coast 
of  Spain  for  about  four  hundred  miles  and  anchored  in  the 
harbor  of  Valencia,  the  first  American  man-of-war  to  visit  a 
Spanish  port  since  the  Spanish-American  War. 

It  had  been  decided  to  divide  the  Naval  Observatory  ex- 
pedition into  three,  sending  two  parties  to  Spain  and  one  to 
Africa.  In  Spain  the  parties  were  located,  one  at  the  edge  of 
the  path  of  totality  at  Puerto  Coeli,  and  the  other  near  the 
central  line  at  Daroca. 

Daroca  is  in  the  heart  of  old  Spain,  about  forty  miles 
from  Saragossa,  and  as  a  railroad  had  been  there  only  four 
years  it  was  a  terra  incognita  for  modern  tourists  —  for 
which  we  were  duly  thankful.  Our  six  weeks'  stay  there 
was  a  happy  commingling  of  hard  work  —  and  there  was 
plenty  of  work  to  do  —  with  pleasant  experiences  in  getting 
acquainted  with  Spanish  life  and  people.  The  site  for  the 
town  is  indeed  a  peculiar  one,  in  a  valley  so  surrounded  by 
hills  that  each  heavy  rain  storm  used  to  flood  the  city,  until 
about  1600,  a  tunnel  was  constructed  through  one  of  the 
hills  to  carry  away  the  waters.  The  tops  of  these  hills  are 
crowned  with  walls  and  forts,  most  of  them  constructed  by 
the  Moors  a  thousand  years  ago,  some  of  them  by  the  Catho- 
lic Spanish  since  that  time.  There  is  one  tower  of  special 
interest,  and  still  in  good  state  of  preservation,  which  is  said 
to  have  been  built  by  the  Romans  before  Saguntum  was 
founded,  and  it  is,  therefore,  more  than  two  thousand  years 
old.  (The  railroad  from  Valencia  passes  through  Saguntum 


FILIPINO  HOMES  BUILT  ON  STILTS  TO  GET  OUT  OP  THE  WET 


HORN-SHAPED  Roors  AND  HANDSOME  WOOD-CARVING  IN  SUMATRA 


THE  ROMAN  FORT  (2200  YEARS  OLD)  AND  MOORISH  FORTIFICATIONS  AT 

DAROCA,  SPAIN 


AN  OLD  AQUEDUCT  AT  DAROCA,  STILL  IN  USE.    LOOKING  SOUTH  TO  THE  VALLEY 


THE    SPANISH    ECLIPSE    OF    1905  185 

where  Hannibal  and  the  Romans  had  their  memorable  fight 
in  B.C.  238.) 

The  Spaniards  received  us  with  open  arms  and  did  every- 
thing in  their  power  to  assist  in  our  work  and  to  make  our 
stay  in  their  midst  as  pleasant  as  possible.  As  no  one  in  the 
place  could  speak  English,  it  was  necessary  to  make  ourselves 
understood  in  their  language. 

To  help  in  the  erection  of  the  temporary  observatory,  six 
sailors  were  sent  in  from  the  Minneapolis,  and  all  hands, 
astronomers  and  sailors,  worked  each  day  from  early  morn- 
ing till  late  at  night,  building  piers,  erecting  telescopes  with 
houses  to  shelter  them,  mounting  spectroscopes,  and  fixing 
up  a  meteorological  observatory.  After  the  carpenters  and 
machinists  had  finished  their  work  of  construction,  it  was 
necessary  for  the  scientists  to  focus  and  adjust,  to  see  that 
everything  was  in  good  working  order,  and  to  make  trial 
photographs.  A  few  days  before  the  eclipse,  the  party  was 
increased  in  size  to  thirty-five,  officers  and  sailors  having 
come  up  from  the  ship  for  the  purpose  of  assisting  in  the 
observations.  Frequent  drills  were  held  in  order  to  fa- 
miliarize each  one  with  his  part  and  thus  to  be  sure  that 
everything  would  go  right  and  that  no  precious  seconds  would 
be  wasted  at  the  time  of  eclipse. 

The  location  of  the  eclipse  camp  was  half  a  mile  south  of 
the  town,  in  the  midst  of  a  beautiful,  fertile  valley.  From 
there,  while  we  worked,  we  could  catch  glimpses  of  scenery 
typical  of  Spain.  The  first  feature  to  attract  one's  atten- 
tion is  the  extremely  barren  aspect  of  the  country,  which 
is  in  sharp  contrast  with  the  garden-like  appearance  of  Eng- 
land. The  hills  of  Spain  were  in  early  times  densely  wooded, 
but  now  are  almost  entirely  devoid  of  trees  and  look  from 
a  distance  as  if  there  were  not  a  particle  of  vegetation  on 
them.  Moreover,  the  rainfall  is  so  slight  that  agricultural 
pursuits  must  rely  upon  irrigation,  and  thus  it  is  only  the  val- 
leys that  are  green  and  cultivated.  In  such  a  valley  along 
the  shores  of  the  little  river  was  our  eclipse  camp  located. 
The  greenest  field  was  decided  upon  as  the  site  of  the  observa- 
tory, and  upon  application  to  its  owner  for  permission  we 
found  that  he  was  quite  satisfied  to  allow  his  plot  of  ground 


186  ECLIPSES    OF    THE    SUN 

to  be  used,  but  thought  some  compensation  should  be  made 
for  the  valuable  crop  of  grass  that  might  possibly  be  raised 
during  the  summer.  On  receipt  of  one  hundred  pesetas,  he 
forthwith  proceeded  to  take  a  fatherly  interest  in  all  of  our 
doing,s,  and  explained  scientific  matters  to  every  one  as  if  he 
had  been  chief  of  the  expedition.  His  field  became  the  cen- 
ter of  interest  in  the  community,  and  people  came  from  all 
sides  to  look  upon  the  strange  doings.  As  a  prominent  trait 
of  the  Spanish  peasant  seems  to  be  a  great  and  overpowering 
curiosity,  we  had  pleny  of  onlookers;  and  when  the  mayor 
and  a  few  of  the  most  prominent  citizens  were  invited  to  look 
at  the  moon  through  our  five-inch  telescope,  we  were  rather 
surprised  —  to  put  it  mildly  —  to  find  over  one  hundred  peo- 
ple turn  up,  when  only  a  half  score  had  been  invited.  Their 
curiosity  took  the  form  only  of  making  each  and  every  one  in 
the  town  intensely  interested  in  what  was  going  on,  and  to 
show  that  interest  they  turned  out  in  force  each  afternoon  to 
see  how  matters  were  progressing.  It  might  be  asked,  what 
their  attitude  was  towards  these  Americans  who  had  so 
lately  beaten  them  in  the  small  war.  Before  the  expeditions 
reached  Spain,  it  was  feared  that  perhaps  there  might  be 
some  friction  on  that  account,  but  these  fears  were  not  real- 
ized. As  a  matter  of  fact,  the  only  person  we  met  who 
seemed  to  have  any  feeling  at  all  in  the  matter  was  a  former 
soldier  in  the  Spanish  army.  He  had  seen  service  in  the 
Philippines,  had  been  captured  and  thrust  into  prison  by  the 
Filipinos,  had  been  rescued  by  the  Americans,  and  as  a  re- 
sult he  had  only  the  kindest  feelings  towards  everything 
belonging  to  the  United  States.  As  for  the  rest  of  the  peo- 
ple, they  seemed  to  have  forgotten  all  about  it,  or  else  they 
did  not  know  there  had  been  a  war,  for  it  must  not  be  for- 
gotten that  only  about  one  quarter  of  the  people  in  Spain  can 
read  and  write. 

After  this  my  third  total  eclipse,  I  can  confidently  say  that 
observations  at  such  a  time  consist  of  much  hard  work  and 
many  nerve-racking  experiences.  The  astronomer  is  never 
on  hand  sufficiently  long  beforehand  to  take  things  quietly 
and  easily,  he  must  work  under  conditions  to  which  he  is 
totally  unused,  and  over  his  head  hangs  the  knowledge  that 


THE    SPANISH    ECLIPSE    OF    1905  187 

everything  must  be  completed  by  a  certain  day  and  a  certain 
hour,  for  the  eclipse  cannot  be  postponed,  and  there  is  no  sec- 
ond trial  in  case  of  failure.  In  addition  to  working  hard  all 
day  as  carpenter  and  instrument  maker,  the  astronomer  must 
stay  up  half  the  night  adjusting  his  instruments  on  stars,  so 
that  during  the  last  few  days  before  the  eclipse  very  few 
hours  of  sleep  each  night  are  obtained.  However,  in  spite  of 
the  many  difficulties  that  were  continually  cropping  up,  the 
mounting  and  adjusting  of  the  instruments  was  practically 
completed  by  August  25,  when  our  observing  party  was 
swelled  in  numbers  by  the  officers  and  men  from  the  Min- 
neapolis. From  then  until  eclipse  day  the  time  was  spent  in 
putting  the  finishing  touches  on  the  work  of  adjustment,  and 
in  having  frequent  drills  in  order  to  insure  that  everything 
would  go  without  a  hitch. 

What  was  the  promise  of  weather  for  the  important  day? 
We  had  been  closely  scrutinizing  the  weather  each  day  to 
see  what  conditions  we  were  to  expect,  and  were  much 
pleased  to  find  that  the  sky  was  usually  clear  just  after  noon, 
the  hour  when  the  eclipse  was  to  occur.  August  29,  how- 
ever, had  been  cloudy  all  day  so  that  on  eclipse  day  we  had 
to  go  to  camp  early  to  test  our  final  adjustments,  go  through 
drills  once  more  and  to  be  sure  that  all  the  apparatus  worked 
smoothly.  The  skies  were  clear  and  our  hopes  for  success 
were  high.  Outside  the  roped-off  enclosure,  the  whole  town 
of  Daroca  was  assembled,  for  it  was  naturally  thought  by 
the  people  that  nowhere  could  the  eclipse  be  seen  so  well 
as  where  the  astronomers  were  located. 

At  11:52  A.M.  a  little  indentation  was  seen  on  the  western 
limb  of  the  sun,  and  the  eclipse  had  begun.  The  skies  were 
clear  with  the  exception  of  a  cloud  here  and  there,  and  our 
most  ardent  wish  was  that  the  clouds  would  leave  the  sun 
clear  for  the  next  couple  of  hours.  For  the  first  hour  that 
the  moon  was  creeping  over  the  sun  there  was  nothing  of 
very  great  moment  to  notice,  but  for  the  next  twenty  min- 
utes till  1:12,  when  the  sun  was  blotted  out,  we  were  each 
of  us  filled  with  expectancy,  for  matters  began  to  take  on 
a  weird  and  unnatural  appearance.  The  little  blotches  of 
light  under  the  trees,  instead  of  being  familiar  circles,  were 


188  ECLIPSES    OF    THE    SUN 

little  crescents,  exact  counterparts  of  the  sun  itself.    The 
darkness  began  to  make  itself  really  felt,  and  without  look- 
ing at  the  sun  one  would  know  that  something  out  of  the 
ordinary  was  happening,  for  the  gloom  did  not  in  the  least 
resemble  that  of  sunset.    A  hush  fell  upon  the  crowd  of  as- 
sembled and  talkative  Spaniards  when,  ten  minutes  before 
totality,  a  big  cloud  drifted  over  the  sun.    Would  this  cloud 
move  away?     Or  were  we  going  to  be  disappointed?     It 
hung  there  for  a  space  of  time  that  seemed  to  be  an  age, 
while  in  reality  it  was  only  five  minutes.    It  was  a  big  scare, 
but  when  that  passed,  with  a  shout  from  us  all,  there  wasn't 
another  cloud  anywhere  to  bother  us.     Seventeen  seconds 
before  the  calculated  time,  with  the  last  disappearing  ray  of 
sunlight,  the  corona  broke  forth  into  view.     What  a  mag- 
nificent sight  it  was  shining  out  with  its  pale,  pearly  light 
for  a  couple  of  diameters  round  the  edge  of  the  sun,  with 
its  streamers  and  brushes  of  delicate  light!     True  to  pre- 
diction, the  corona  was  almost  square  in  shape,  and  was  not 
at  all  alike  in  appearance  to  the  other  coronas  the  writer  had 
seen  in  1900  and  1901,  with  their  long  fish-tail  extensions 
along  the  sun's  equator  and  short-curved  streamers  near  the 
sun's  poles.    In  the  upper  left  hand  quadrant,  huge  red  flames 
sixty  thousand  miles  high  could  be  seen  with  the  naked  eye, 
and  these  with  a  closer  view  with  the  telescope  resolved  them- 
selves into  a  forest-like  structure.    Close  to  the  sun  the  corona 
was  very  bright,  in  fact  so  brilliant  that  the  eye  was  not  read- 
ily able  to  take  in  all  the  details  of  the  faint  streamers.    As  a 
pictorial  effect,  without  the  long  equatorial  extensions,  this 
corona  was  much  inferior  to  the  two  last  ones  seen.    Still  it 
was  a  magnificent  sight,  and  we  were  more  than  thankful  for 
having  clear  skies  for  making  our  observations. 

When  totality  first  started  we  were  each  and  all  of  us  much 
too  busy  to  take  very  much  notice  of  our  immediate  sur- 
roundings or  even  of  the  corona  itself.  We  could  not  help 
becoming  aware  that  our  Spanish  onlookers  outside  the  ropes 
were  appreciating  the  show  in  the  skies  provided  for  them 
without  expense.  From  the  noise  made  each  one  seemed  to 
be  telling  his  neighbor  at  the  top  of  his  voice  just  how  it  hap- 
pened and  what  there  was  worth  seeing,  and  this  in  spite  of 


THE    SPANISH    ECLIPSE    OF    1905  189 

the  fact  that  the  mayor  of  Daroca  had  generously  provided 
half  a  dozen  members  of  the  civil  guard  to  preserve  order  and 
keep  quiet.  For  the  first  half  minute  the  din  was  so  great 
that  it  was  impossible  to  hear  the  seconds  counted,  or  to  know 
exactly  when  to  begin  and  end  the  exposures  of  the  photo- 
graphs. When  the  Spaniards  had  quieted  down,  after  their 
first  outburst,  all  that  was  heard  in  the  eclipse  camp  was  the 
steady  count  of  the  observer  calling  out  the  seconds  as  they 
passed,  the  quiet  words  of  the  observers  giving  commands  to 
their  assistants  and  the  click,  click  of  the  various  pieces  of 
apparatus  as  exposures  were  made  and  plate  holders  removed. 
Everything  passed  off  without  a  hitch,  and  with  the  first  re- 
appearance of  the  sun  our  work  was  over  and  we  could  take  a 
long  breath. 

We  had  been  favored  with  clear  skies.  How  many  others 
were  equally  fortunate?  It  did  not  take  us  long  to  find  out, 
for  the  Spanish  government  had  installed  right  in  our  camp 
a  telegraph  office,  and  for  fifteen  days  no  less  than  three 
operators  were  at  our  service  to  send  and  receive  our  mes- 
sages; and  for  this  not  a  single  cent  of  money  was  asked 
or  expected.  It  was  found  that  fifty  miles  to  the  west  of 
us,  at  Alhama,  where  were  the  observers  from  the  Lick  Ob- 
servatory under  Professor  Campbell,  there  were  thin  clouds, 
while  one  hundred  miles  to  the  east  along  the  Mediterranean 
coast,  the  Englishmen  were  even  more  unfortunate  in  hav- 
ing the  clouds  denser.  In  the  northeastern  part  of  Spain  at 
Burgos,  more  astronomers  were  located  than  at  any  one  place, 
and  here  too  was  King  Alfonso  of  Spain.  Five  minutes  before 
totality  it  was  pouring  rain,  but  as  if  by  a  miracle  a  little  blue 
patch  of  sky  appeared,  and  the  eclipse  was  seen  under  per- 
fect conditions.  The  weather  along  the  eclipse  track  was: 
in  Labrador,  cloudy  (no  observations  made)  ;  in  Spain,  cloudy 
and  clear;  in  the  islands  of  the  Mediterranean,  cloudy;  on 
the  coast  of  Africa,  slightly  cloudy;  but  farther  inland  and 
along  the  rest  of  the  eclipse  track  the  skies  were  perfect.  All 
three  parties  of  the  Naval  Observatory  were  fortunate  in  hav- 
ing their  work  unhindered  by  a  single  cloud. 

My  own  work  was  entirely  spectroscopic.     The  photo- 
graphic plates  were  developed  within  the  walls  of  the  col- 


190  ECLIPSES    OF    THE    SUN 

lege  of  Daroca,  and  in  the  long  hours  necessary  for  this  work 
I  was  greatly  encouraged  and  assisted  by  my  good  friend 
the  rector  of  the  college,  Padre  Felix  Alvirez.  Daily  inter- 
course with  this  reverend  father  endeared  him  to  me  very 
much;  and  Senors  Lorente,  Soria  and  Padre  Felix  made  my 
stay  in  Daroca  one  of  the  most  interesting  spots  of  my  whole 
life  by  the  kindness  with  which  they  bore  my  imperfect  Span- 
ish, by  the  interesting  bits  of  history  they  told  of  Daroca,  and 
by  the  deep  insight  each  gave  into  the  courtesy  of  a  Spanish 
gentleman's  heart.  The  developed  photographs  of  the  flash 
spectrum  showed  exquisite  definition  over  a  wide  range  in 
wave-lengths.  Evidently  I  had  stored  up  for  myself  the  ma- 
terial for  months  and  years  of  intensive  study,  the  results  of 
which  will  be  told  in  Chapter  XVI. 


i 


CHAPTER  XII 

THE  AMERICAN  ECLIPSE  OF  1918 

HIRTEEN  years  after  the  splendid  observing  con- 
ditions of  1905  in  Spain  I  witnessed  my  fourth  total 
eclipse  of  the  sun,  again  as  a  member  of  the  ex- 
pedition representing  the  United  States  Naval  Observatory. 
On  June  8,  1918,  the  shadow  of  the  moon  touched  the  earth's 
surface  on  the  Pacific  Ocean,  far  south  of  Japan.  Owing  to 
the  revolution  of  the  moon  about  the  earth  and  to  the  rota- 
tion of  the  earth  on  its  axis,  the  shadow  crossed  the  Pacific 
Ocean  at  a  speed  of  over  a  thousand  miles  per  hour.  It  was 
well  after  noon  before  the  shadow  reached  the  American  con- 
tinent, and  the  eclipse  began  in  the  state  of  Washington. 
Here  the  width  of  the  shadow  was  only  sixty  miles,  so  that 
only  those  fortunate  enough  to  be  within  this  narrow  track 
were  able  to  see  the  eclipse  in  its  totality.  The  eclipse  passed 
southeasterly  through  Washington,  Oregon,  Idaho,  Wyo- 
ming and  Colorado  in  succession.  In  Colorado,  the  shadow 
had  dwindled  to  forty  miles  in  width.  After  passing  through 
some  of  the  central  states,  the  shadow  left  the  United  States 
at  Florida  and  left  the  earth's  surface  in  the  Atlantic,  off  the 
coast  of  the  Bahama  Islands. 

The  eclipse  was  seen  almost  exclusively  from  the  United 
States,  and  so  it  will  be  known  as  the  American  Eclipse  of 
1918.  Since  more  than  half  the  civilized  world  was  in  the 
grip  of  the  tremendous  war,  it  was  necessary  for  American 
astronomers  in  the  year  1916  and  early  in  1917  to  make 
their  plans  to  insure  that  this  eclipse  should  be  well  ob- 
served. Before  our  own  country  had  become  involved  in 
the  war,  Congress  had  been  asked  for,  and  had  made,  a 
special  appropriation  to  defray  the  expense  of  equipment 
and  travel  for  the  party  from  the  U.  S.  Naval  Observatory. 

In  order  to  help  the  astronomers  of  the  country  to  make 

191 


192  ECLIPSES    OF    THE    SUN 

as  intelligent  a  choice  of  an  eclipse  site  as  possible,  the  Naval 
Observatory,  in  1917,  had  prepared  a  large  scale  map  of  the 
United  States  showing,  among  other  things,  railroad  lines, 
contour  lines  and  the  location  of  towns  within  the  eclipse 
track.  The  city  of  Baker  in  eastern  Oregon  seemed  to  be  the 
ideal  spot  for  the  government  party.  The  question  of  clear 
skies  was  the  all-important  one  for  the  proper  location  of  an 
eclipse  party,  but,  fortunately,  the  U.  S.  Weather  Bureau 
had  a  regularly  equipped  station  at  Baker,  and  a  record  of 
many  years7  continuous  observations  seemed  to  be  the  ideal 
method  of  securing  the  desired  information  regarding  the 
probabilities  of  good  conditions  on  the  day  of  June  8.  As 
the  Weather  Bureau  promised  an  absence  of  clouds  and  rain 
with  an  abundance  of  clear  skies,  Baker  was  chosen  with  the 
great  hope  that  it  would  live  up  to  its  good  reputation  in  the 
matter  of  weather. 

We  were  in  Baker  exactly  six  weeks  before  eclipse  day, 
and  the  time  was  none  too  long.  The  apparatus  was  sent  for- 
ward by  through  freight,  and  though  we  greatly  feared  de- 
lays, it  arrived  safely  the  second  day  after  our  own  arrival. 
To  assist  in  the  work  of  erecting  the  apparatus,  the  superin- 
tendent of  the  Naval  Observatory  had  requested  the  services 
of  five  sailors  from  the  U.  S.  Naval  Station  at  Bremerton, 
Washington,  who  were  in  charge  of  a  chief  petty  officer.  The 
sailors  were  carpenters  and  machinists  who  assisted  the  as- 
tronomers in  splendid  style  so  that  ten  days  before  the  eclipse, 
when  the  balance  of  the  party  began  to  arrive,  the  apparatus 
was  all  erected  and  partially  adjusted,  and  there  remained 
only  the  perfecting  of  the  adjustments  in  order  to  be  ready 
for  the  all-important  day  of  the  eclipse. 

For  direct  photography  of  the  corona,  the  largest  camera 
was  one  of  sixty-five  feet  focal  length  arranged  horizontally, 
the  light  from  the  eclipsed  sun  being  reflected  by  a  coelostat 
mirror.  The  spectroscopic  work  of  the  Naval  Observatory 
party  called  for  the  use  of  three  concave  gratings,  each  used 
objectively  without  slit.  The  largest  instrument  was  a 
twenty-one-foot  Rowland  grating  of  six  inches  aperture  and 
15,000  lines  per  inch.  This  had  a  spectrum  specially  bright 
in  the  first  order  on  one  side,  the  grating  being  kindly  loaned 


THE    AMERICAN    ECLIPSE    OF    1918  193 

by  Professor  J.  S.  Ames.  Photographic  films  two  by  twenty- 
four  inches  were  used,  and  it  was  planned  to  work  from  the 
extreme  ultra-violet  as  far  to  the  red  as  the  length  of  the 
films  would  permit.  The  second  concave  grating  was  of  ten 
feet  radius  and  15,000  lines  per  inch,  the  grating  belonging 
to  the  Naval  Observatory.  This  was  used  in  the  first  order 
and  gave  the  same  dispersion  as  the  instrument  employed  in 
Daroca,  Spain,  in  1905.  By  the  use  of  special  emulsions 
kindly  prepared  by  Dr.  C.  E.  K.  Mees  of  the  Eastman  Kodak 
Company,  an  attempt  was  made  to  photograph  farther  to  the 
red  end  than  in  1905.  The  third  one  was  a  very  short  focus 
grating  of  two  meters  radius  and  of  six  inches  aperture  be- 
longing to  the  Astrophysical  Observatory  of  the  Smithsonian 
Institution.  This  grating  gave  very  brilliant  spectra  but 
with  little  dispersion.  The  films  for  this  spectrograph  were 
stained  with  dicyanine. 

Fortunately  for  the  work  of  preparation,  and  true  to  the 
prediction  of  the  U.  S.  Weather  Bureau,  no  rain  fell  during 
the  entire  stay  of  the  astronomical  party  in  Baker.  Accord- 
ing to  the  "  oldest  inhabitant/'  the  season  was  unusually  dry 
even  for  eastern  Oregon.  By  some  mysterious  force  un- 
known to  the  astronomers,  the  eclipse  seemed  to  exert  some 
potent  influence  over  the  weather.  At  any  rate,  it  was  as- 
serted by  many  of  the  rural  papers  that  no  rain  could  be 
expected  until  the  eclipse  was  over.  But  if  an  absence  of 
rain  was  experienced  there  was  no  lack  of  clouds,  nor  were 
the  clear  skies  we  had  been  led  to  expect  afforded  us.  As 
the  time  for  the  eclipse  drew  nearer,  the  continued  appear- 
ance of  clouds  began  to  cause  anxiety  among  us.  Would 
they  interfere  with  the  eclipse,  and  at  the  last  moment  make 
all  the  weeks  of  careful  preparation  of  no  account?  If  this 
had  indeed  happened,  it  would  not  have  been  the  first  event 
of  the  kind.  Unfortunately  for  the  astronomer,  his  work 
is  always  at  the  mercy  of  the  clouds  and  the  weather.  But 
to  have  the  whole  work  fail  through  clouds  at  the  time  of 
the  few  precious  minutes  of  the  total  eclipse  —  that  is  in- 
deed,the  keenest  sort  of  disappointment!  Some  astronomers 
seem  to  be  always  unlucky  and  always  experience  cloudy 
weather  on  their  eclipse  expeditions,  while  on  the  other  hand 


194  ECLIPSES    OF    THE    SUN 

others  are  always  lucky,  and  sometimes  after  all  hope  is 
abandoned,  a  rift  will  appear  in  the  clouds  and  the  eclipse 
at  totality  be  seen  in  all  its  glory.  Would  we  at  Baker  be 
lucky  or  unlucky,  would  the  clouds  interfere  or  not?  Nearly 
all  the  days  spent  in  Baker,  according  to  the  classification 
of  the  U.  S.  Weather  Bureau,  were  actually  clear.  A  "  clear  " 
day  is  not  necessarily  cloudless  from  morning  till  night,  but 
rather  one  when  the  "  sky  averages  three-tenths  or  less  ob- 
scured, from  sunrise  to  sunset."  Clouds,  however,  gathered 
almost  every  day  shortly  after  noon,  and  this  condition  was 
usually  accompanied  by  very  high  winds  that  at  times  rose 
to  the  strength  of  a  mild  gale.  The  eclipse  was  to  occur  dur- 
ing the  middle  of  the  afternoon,  and  at  this  time  of  day  the 
skies  were  generally  overcast.  These  same  conditions  pre- 
vailed over  the  whole  of  the  western  United  States  along  the 
path  where  the  astronomers  were  located.  It  was  well  to  be 
an  optimist  under  such  conditions  of  sky,  for  the  pessimist 
became  more  and  more  wretched  as  the  day  of  the  eclipse 
drew  near  and  his  law  of  averages  showed  him  the  almost 
certain  chance  of  thinly  clouded  sky  during  the  total 
eclipse. 

Fortunately,  so  far  in  my  eclipse  experiences  I  had  been 
among  the  lucky  astronomers.  In  1900,  at  my  first  eclipse, 
the  weather  was  ideal  —  not  a  single  cloud  in  the  whole 
sky.  In  1901, 1  was  a  member  of  a  rather  large  party  which 
traveled  half  way  round  the  world,  of  which  only  four  of  a 
total  of  thirteen  saw  the  eclipse,  the  other  nine  witnessing  the 
eclipse  eclipsed  by  clouds.  I  was  one  of  the  fortunate  four. 
Again,  in  1905,  there  were  many  clouds  which  spoiled  the 
researches  of  many  parties.  At  Daroca,  in  Spain,  a  few  min- 
utes before  totality  a  dense  cloud  covered  the  sun,  but  it 
cleared  away  before  the  all-important  time  and  the  total 
phase  was  seen  through  a  brilliantly  clear  sky.  Three  lucky 
chances  out  of  three  made  a  fine  average.  The  hope  was 
that  June  8  would  make  it  four  out  of  four! 

By  May  30,  the  whole  party  had  assembled  in  Baker.  A 
full  week  was  given  up  to  the  final  adjustments,  and  to  the 
drills  that  were  to  play  such  an  essential  part  in  the  work 
on  eclipse  day.  During  the  partial  phases  of  the  eclipse, 


THE  AMERICAN  ECLIPSE  OF  191  s         195 

very  few  observations  of  importance  were  to  be  made;  all 
observations  of  value  came  during  the  period  of  totality 
which  lasted  for  one  hundred  and  twelve  brief  seconds.  If 
a  slide  of  a  plate  holder  should  stick  in  place  so  that  it  could 
not  be  removed,  or  a  lens  were  not  uncapped  at  the  proper 
time  so  as  to  let  in  the  light,  the  whole  work  of  an  instru- 
ment might  come  to  naught.  On  each  day  of  the  week  pre- 
ceding June  8,  drills  were  gone  through  several  times,  in 
the  morning  and  again  in  the  afternoon.  These  drills  were 
so  well  carried  out  that  on  eclipse  day  each  and  every  one 
performed  excellently  the  task  allotted  to  him  with  the  re- 
sult that  everything  passed  off  without  a  single  hitch. 

As  the  days  in  June  progressed  towards  the  eighth,  there 
was  an  air  of  excitement  as  each  astronomer  grew  more 
keyed  up  to  the  task  before  him.  Would  Saturday  be  clear? 
But  more  especially,  would  the  two  minutes  from  4:04  to 
4:06  P.M.  be  clear?  The  skies  were  anxiously  watched  dur- 
ing the  last  days,  but  alas !  almost  every  day  at  eclipse  hour 
they  were  overcast.  The  optimist  reasoned  that  if  it  were 
cloudy  all  the  days  before  June  8,  then  on  eclipse  day  per- 
fect weather  would  surely  be  forthcoming;  while  the  pessi- 
mist on  the  other  hand  argued  that  so  many  cloudy  days 
meant  still  one  more  of  the  same  character,  so  there  would 
be  no  use  trying  to  do  anything. 

Saturday,  June  8,  dawned  with  the  sky  overcast  with  thin, 
filmy  clouds.  The  sun  was  well  visible  through  these  clouds, 
however,  and  it  was  possible  to  examine  again  the  focus  that 
had  been  obtained  with  the  spectroscopes  and  with  a  touch 
here  and  a  touch  there  to  decide  that  everything  was  in 
perfect  condition.  During  the  morning  the  drills  were  again 
practised,  and  these  seemed  to  promise  success.  The  weather 
during  the  six  weeks  had  not  held  up  the  work,  and  every- 
thing that  thought  and  work  could  do  seemed  now  to  have 
been  accomplished.  The  astronomers  who  had  been  on  the 
ground  for  the  whole  six  weeks  of  preparation  had  the  pleas- 
ant consciousness  that  all  of  their  allotted  tasks  had  been 
completed,  that  every  little  detail  had  been  thought  of  and 
that  perfect  success  would  certainly  crown  their  efforts  if  only 
the  clouds  would  clear  away.  But  during  the  course  of  the 


196  ECLIPSES    OF    THE    SUN 

morning/ the  clouds  grew  thicker  instead  of  thinner,  and  it 
did  indeed  seem  as  if  there  was  little  chance  of  a  clear  sky. 

The  first  contact  was  to  take  place  at  2:30  P.M.  Shortly 
after  noon,  the  city  of  Baker  took  upon  itself  the  aspect  of 
a  holiday.  Though  the  day  was  Saturday,  all  stores  were 
closed  from  three  until  five  in  the  afternoon  so  that  every- 
one should  have  a  chance  to  see  the  phenomenon.  Naturally 
everyone  in  Baker  wished  to  go  to  the  eclipse  site  at  the  Fair 
Grounds,  to  watch  the  astronomers  at  work.  At  the  eclipse 
in  Spain,  this  had  been  permitted  with  the  result  that  the 
whole  town  had  assembled,  each  inhabitant  jostling  his 
neighbor  to  get  as  close  as  possible,  and  each  apparently 
talking  at  the  top  of  his  lungs,  with  the  result  that  such  a 
din  arose  when  the  eclipse  became  total  that  it  was  impos- 
sible to  hear  the  seconds  counted  off  to  give  warning  to  the 
astronomers  when  to  change  their  plate  holders. 

In  order  that  this  might  not  happen  again,  the  residents 
of  Baker  were  told  that  the  gates  of  the  Fair  Grounds  would 
be  closed,  and  that  absolutely  no  one  would  be  admitted 
within  the  enclosure,  and  the  mayor  of  the  city  sent  a  guard 
of  Boy  Scouts  to  see  that  these  orders  were  obeyed.  Most 
of  the  townspeople  repaired  to  the  hills  to  the  southeast  of 
the  city  from  which  there  could  be  obtained  a  fine  view  of 
the  valley  and  the  Elkhorn  range,  and  they  were  directed 
to  look  especially  for  the  shadow  of  the  moon  which  would 
come  across  the  landscape  at  the  speed  of  thirty  miles  a 
minute  or  1800  miles  per  hour. 

No  appreciable  improvement  in  the  skies  was  observed 
from  noon  to  the  time  of  first  contact.  Through  a  thin  patch 
in  the  clouds,  Mr.  Hammond,  using  the  five-inch  visual  tele- 
scope, observed  the  beginning  of  the  eclipse  and  made  a 
record  of  it.  The  clouds  if  anything  became  thicker  after 
this  so  that  at  three  o'clock  it  was  impossible  to  see  even 
where  the  sun  was.  Little  thin  rifts  appeared  at  times,  so 
that  it  was  possible  to  see  the  moon  encroaching  on  the  face 
of  the  sun.  At  three  thirty,  a  patch  of  brilliantly  blue  sky 
was  seen  off  to  the  northwest  and  as  the  precious  minutes 
dragged  along  it  became  evident  that  the  clouds  were  moving 
in  such  a  way  that  it  was  quite  possible  that  the  blue  patch 


THE    AMERICAN    ECLIPSE    OF    1918  197 

would  reach  the  sun  in  time  for  totality.  Fifteen  minutes 
before  the  total  phase  the  clouds  were  so  dense  that  had 
totality  occurred  then,  the  scientific  results  would  have  been 
nothing;  but  the  blue  sky  was  coming  nearer  and  it  might 
arrive  in  time. 

Without  looking  at  the  sky,  one  realized  that  something 
unusual  was  happening.  The  light  of  the  sun  became  so  fee- 
ble that  even  the  birds  felt  the  unnatural  aspect  of  things 
and  sang  their  songs  as  if  they  were  going  to  rest.  The  cocks 
in  the  nearby  farm  crowed.  The  wind,  which  was  ordinarily 
blowing  at  this  hour,  was  quiet.  All  nature  was  hushed. 
Even  the  seasoned  astronomers  who  had  seen  two  or  three 
eclipses  before  felt  the  thrill  of  the  unusual  spectacle.  And 
still  the  question  was,  would  the  clouds  clear  away  in  time? 

At  five  minutes  before  totality  the  warning  signal  was 
given  by  Chief  Petty  Officer  Welsch  of  the  U.  S.  Navy  who 
was  to  watch  the  chronometer  and  count  the  seconds.  This 
signal  summoned  each  man  to  his  post.  One  last  look  was 
given  to  the  apparatus  to  see  that  everything  was  in  place, 
the  plate  holders  were  adjusted,  —  and  then  we  waited. 
"  Two  minutes  "  before  was  called  out,  and  then  "  one  min- 
ute/' still  again  "  thirty  seconds  "  before  the  expected  time  of 
totality.  The  clouds  by  this  time  had  thinned  considerably, 
the  patch  of  blue  sky  was  only  a  short  distance  away.  The 
plan  had  been  that  after  the  signal  of  "  thirty  seconds,"  there 
should  be  nothing  said  until  the  word  "  Go  "  told  that  the 
total  eclipse  had  begun.  I  was  to  watch  for  this  with  a  pair 
of  binoculars,  before  one  glass  of  which  a  direct  vision  spec- 
troscope had  been  arranged.  This  was  the  plan  followed  in 
Spain  with  complete  success.  But  due  to  the  thin  clouds  at 
the  beginning,  it  was  impossible  to  see  the  spectrum  lines 
with  the  spectroscope,  and  the  signal  "  Go  "  was  actually 
given  by  Mr.  Hammond  who  was  using  the  five-inch  tele- 
scope. No  sounds  disturbed  the  work  of  the  party  except 
the  call  of  the  seconds  as  the  time  passed  and  the  brief  words 
of  command  and  shift  of  plate  holders  as  each  member  of  the 
party  did  his  allotted  task.  Ten  seconds  after  totality  com- 
menced, the  clouds,  thin  at  the  beginning,  had  still  further 
thinned,  and  at  mid- totality  the  conditions  were  even  further 


198  ECLIPSES    OF    THE    SUN 

improved.  What  a  gorgeous  spectacle  then  met  the  eye! 
The  sun  was  now  in  a  very  thin  wisp  of  cloud  with  blue  sky 
on  either  side.  Although  the  cloud  would  undoubtedly  de- 
tract from  the  scientific  results,  still  it  greatly  enhanced  the 
pictorial  effect.  The  corona  could  be  seen  stretching  for  a 
short  distance  from  the  sun's  edge,  but  most  remarkable  of 
all  were  three  great  tongues  of  flame,  one  immediately  at  the 
top  of  the  sun,  one  on  the  left  hand  edge,  and  still  a  larger 
one  on  the  right  edge  of  the  sun.  These  shone  with  a  brilliant 
scarlet  light,  and  made  the  eclipse  of  1918  memorable  as  the 
eclipse  of  color.  As  the  end  of  totality  approached  the  thin 
clouds  became  still  thinner,  —  and  two  minutes  after  the 
eclipse  was  over  the  sun  had  reached  the  blue  patch  of  sky. 
If  the  eclipse  had  occurred  only  two  minutes  later,  or  if  the 
party  had  been  only  half  a  mile  to  the  northwest,  the  sky 
conditions  would  have  been  perfect!  If,  as  I  have  already 
said,  the  eclipse  had  taken  place  fifteen  minutes  earlier,  the 
scientific  results  would  have  been  nothing  at  all.  The  op- 
timists had  won  out. 

We  had  indeed  been  fortunate.  But  farther  west  at 
Goldendale,  Washington,  where  the  Lick-Crocker  party  was 
located,  a  change  of  weather  had  happened  which  amounted 
almost  to  a  miracle.  The  account  by  Professor  Campbell 
runs  as  follows:  "  The  total  phase  of  the  eclipse  occurred 
at  2:57,  local  mean  time.  By  great  good  luck  a  small  rift 
in  the  clouds  formed  mostly  at  the  right  place  and  right 
time.  The  clouds  uncovered  the  sun  and  its  immediate  sur- 
roundings less  than  a  minute  before  totality  became  com- 
plete, and  the  clouds  again  covered  the  sun  less  than  one 
minute  after  the  total  phase  had  passed.  The  small  clear 
area  was  very  blue  and  the  atmosphere  was  tranquil."  * 

The  developed  photographs  exhibit  the  painstaking  care 
of  the  astronomers  in  procuring  the  precise  focus,  with  the 
result  that  all  of  the  photographs  show  exquisite  definition. 
The  thin  clouds  did  not  interfere  at  all  with  the  details  of 
the  prominences  or  flames  surrounding  the  sun.  Those 
taken  with  the  sixty-five-foot  camera  exhibit  the  promi- 
nences in  splendid  detail  on  a  scale  where  the  sun  is  more 

1  Lick  Observatory  Bulletin,  10,  2,  1918. 


THE    AMERICAN    ECLIPSE    OF    1918  199 

than  seven  inches  in  diameter.  The  longer  exposures  for 
procuring  the  extensions  of  the  corona  were  not  quite  so  suc- 
cessful, since  the  thin,  fleecy  clouds  cut  down  the  fainter 
streams  of  coronal  light.  The  smaller  cameras  showed  the 
same  results  as  the  larger  ones  —  splendid  detail  in  the  in- 
ner corona,  but  the  corona  not  of  very  great  extent.  All 
the  photographs  unite  in  showing  many  polar  rays,  and  they 
also  exhibit  some  plumed  arches  of  great  beauty.  The  corona 
appeared  to  be  of  the  sun-spot  maximum  type,  but  with  more 
polar  streamers  than  were  expected. 

The  spectroscopes  procured  photographs  of  exquisite  defi- 
nition, but  these  photographs  suffered  greatly  owing  to  clouds 
which  cut  down  the  amount  of  exposure  that  at  best  is  none 
too  great. 

What  was  perhaps  the  most  interesting  piece  of  scientific 
work  accomplished  at  the  1918  eclipse  owes  its  conception 
to  Mr.  Edward  D.  Adams,  of  New  York,  who  has  shown  his 
great  interest  in  science  by  the  founding  of  the  Ernest  Kemp- 
ton  Adams  fellowship  which  is  awarded  each  year  by  Co- 
lumbia University  for  researches  in  the  domain  of  pure  sci- 
ence. Upon  becoming  a  member  of  the  United  States  Naval 
Observatory  party,  Mr.  Adams  took  upon  himself  the  re- 
sponsibility of  trying,  by  some  method,  by  photography,  by 
a  drawing,  or  by  a  painting,  to  procure  a  reproduction  which 
would  show  the  beauties  of  the  corona,  and  which  should  be 
true  not  only  as  to  form  but  more  especially  as  to  color.  Un- 
fortunately for  science,  it  is  impossible  to  obtain  a  satisfac- 
tory representation  of  the  corona  and  the  sun's  surround- 
ings by  photography.  The  corona  is  very  brilliant  near  the 
edge  of  the  sun,  but  the  intensity  fades  very  rapidly.  The 
eye  can  take  cognizance  of  the  details  in  spite  of  the  great 
changes  in  brilliance,  but  not  so  the  photographic  plate.  To 
obtain  the  faint  extensions  of  the  corona  which  are  readily 
visible  to  the  naked  eye,  a  comparatively  long  exposure  is 
necessary.  This  long  exposure  causes  so  much  overexposure 
in  the  brighter  inner  regions  of  the  corona  that  all  detail 
there  is  lost  by  being  burnt  out.  Short  exposures  give  us  the 
inner  corona  in  exquisite  detail,  but  the  outer  corona  is  then 
lost  through  shortness  of  exposure.  Many  attempts  have 


200  ECLIPSES    OF    THE    SUN 

been  made  to  cut  down  the  relative  exposure  by  means  of 
mechanical  devices  —  but  none  of  these  have  been  entirely 
successful.  Heretofore,  the  only  success  in  representing  the 
corona  has  been  obtained  by  taking  photographs  with  differ- 
ent times  of  exposure  and  with  different  cameras  in  order  to 
procure  photographs  with  detail  both  in  the  inner  and  brighter 
parts  of  the  corona,  and  in  the  fainter  outlying  portions. 
After  the  eclipse  is  over,  a  composite  drawing  is  usually  made 
from  the  examination  of  different  photographs.  This  method 
has  given  several  satisfactory  drawings,  but  they  still  have 
left  much  to  be  desired.  However  perfect  they  may  have 
been  as  drawings,  they  took  no  note  of  color.  Mr.  Adams 
took  upon  himself  the  task  of  finding  the  right  man  to  draw 
and  paint  the  corona.  Color  photography  could  not  help  out 
in  procuring  the  right  color,  and  there  was  left  only  the  pos- 
sibility of  finding  an  artist  who  would  have  the  true  scientific 
spirit,  and  who  could  combine  an  accurate  sense  of  form  with 
a  refined  perception  of  color.  Mr.  Adams  was  successful  in 
finding  Mr.  Howard  Russell  Butler,  a  portrait  painter  of 
note,  who  has  developed  a  shorthand  method  of  noting  both 
form  and  color. 

During  the  eclipse,  Mr.  Butler  sat  on  a  lofty  perch  over- 
looking the  eclipse  instruments,  and  from  which  he  could 
obtain  a  fine  view  of  the  sun.  The  task  he  had  taken  to 
himself  was  no  small  one.  As  a  portrait  painter  he  usually 
asked  for  ten  or  twelve  sittings  of  two  hours  each:  now  he  was 
asked  to  render  his  subject  in  112  seconds.  And  moreover 
this  was  the  first  corona  he  had  ever  seen ! 

The  methods  followed  by  Mr.  Butler  in  painting  the  co- 
rona have  been  described  by  him  in  Natural  History,  IQ, 
244-271,  1919,  and  reprinted  as  Vol.  II,  part  6,  of  the  Pub- 
lications of  the  Leander  McCormick  Observatory.  An  ab- 
breviated summary  of  his  description  is  herewith  given : 

"  The  method  of  working  finally  adopted  may  be  called  a 
shorthand  method.  It  was  to  have  a  sheet  of  white  card- 
board on  the  easel  with  a  series  of  concentric  circles  and  radii 
drawn  upon  it  in  advance.  One  of  these  circles  was  to  have 
the  same  diameter  as  the  photographs  of  the  moon  to  be 
taken  in  the  sixty-five-foot  camera,  namely,  seven  and  three- 


THE    AMERICAN    ECLIPSE    OF    1918  201 

eighths  inches.  There  was  to  be  an  inner  circle  of  half  this 
diameter  and  outer  circles  whose  diameters  were  respectively 
one  and  one  half,  two,  and  two  and  one  half  times  that  of  the 
inner  circle.  I  expected  to  use  the  seven  and  three-eighths 
inch  diameter,  and  did  actually  use  it,  but  I  was  thus  pre- 
pared, in  case  of  an  unexpectedly  extended  corona,  to  reduce 
the  scale  to  one  half  and  get  everything  on  the  cardboard.  In 
front  and  beneath  my  cardboard  was  a  finished  sample  pic- 
ture of  a  corona,  painted  in  advance  as  I  expected  it  would 
appear,  and  my  plan  was  to  indicate  by  initials  at  points  on 
my  cardboard  the  variations  of  color  from  this  picture ;  thus 
b  was  to  mean  a  variation  toward  blue  from  the  sample  pic- 
ture, and  y  more  toward  yellow.  I  wrote  out  the  procedure 
as  follows  and  tacked  it  alongside  the  easel.  Practice  enabled 
me  to  allot  a  certain  number  of  seconds  to  each  item. 

Procedure  Seconds 

Note  value  and  color  of  sky 10 

Draw  value  line  on  moon i  o 

Note  colors  of  moon 10 

Draw  outline  of  corona 20 

Use  Zeiss  binoculars 20 

Record  positions  of  prominences 10 

Note  color  and  value  of  prominences 10 

Note  colors  and  values  of  corona,  etc 20 


no 


"  Then  my  plan  was  to  paint  a  first  picture  from  this  result- 
ing memorandum,  while  the  impression  was  vivid,  and  as  soon 
as  there  was  sufficient  light  to  proceed  by. 

"  While  disappointed  in  not  seeing  the  corona  in  a  cloud- 
less sky,  the  thin  veil  had  its  advantage  from  the  artist's 
standpoint.  It  added  mystery  and  the  effect  was  picturesque. 
The  brilliant  corona  burned  through  the  thin  veil  as  if  it  were 
not  there.  Probably  only  the  outside  edges  of  the  corona 
were  affected. 

"  On  the  tenth,  the  photograph  negatives  were  shown  to 
me.  Those  of  the  sixty-five-foot  camera  were  seven  and 
three-eighths  inches  in  diameter,  the  others  considerably 
smaller.  I  now  saw,  in  minute  detail,  the  two  prominences 
which  I  had  recorded  and  the  mighty  cyclone  which  had 


202  ECLIPSES    OF    THE    SUN 

been  increasingly  revealed  as  the  eclipse  neared  its  end,  be- 
cause of  the  direction  of  the  moon's  motion.  There  were 
many  other  minor  prominences. 

"  I  now  made  careful  drawings  of  these  prominences  from 
the  negatives  and  of  the  variations  in  shading  of  the  sur- 
rounding corona.  Many  arches  were  found  springing  over 
the  prominences,  and  a  few  rifts  of  dark  channels  radiating 
from  the  limb  but  never  coming  very  close  to  it.  The  nega- 
tives showed  very  clearly  the  hairy  polar  rays,  not  always 
radial  in  direction,  and  the  beginning  of  a  wing  springing 
from  the  upper  right-hand  limb  of  the  sun. 

"  Three  paintings  were  made,  the  first  immediately  after 
the  eclipse,  the  second  on  the  succeeding  day  and  the  third 
after  all  data  had  been  secured.  This  final  painting  is  the 
one  reproduced  on  page  60."  The  original  size  of  the  paint- 
ing is  49  x  33^-  inches. 

One  of  Mr.  Butler's  paintings  has  been  presented  to  the 
American  Museum  of  Natural  History  in  New  York  by  Mr. 
Edward  D.  Adams.  This  canvas  is  mounted  in  the  Astro- 
nomical Room  which  is  kept  darkened  but  with  the  corona 
painting  illuminated  by  indirect  lighting.  Those  who  have 
been  privileged  to  see  this  painting  have  pronounced  it  a 
thing  of  rare  beauty.  The  astronomers  who  saw  the  1918 
eclipse  and  who  have  seen  the  picture  look  upon  it  as  a  marvel 
of  perfection,  true  both  as  to  form  and  color,  a  great  work 
of  art  which  has  the  added  advantage  of  being  scientifically 
accurate.  In  the  same  room  of  the  great  Museum  are  paint- 
ings of  the  1923  and  192  5  coronas,  also  from  the  skilled  hands 
of  Mr.  Butler,  the  three  coronas  forming  a  triptych  with  in- 
teresting contrasts.  A  reproduction  of  the  1923  painting  is 
found  facing  page  204  and  that  of  1925  as  the  frontispiece 
illustration. 

The  results  from  the  expeditions  of  1919  and  1922  which 
were  devoted  mainly  to  tests  of  the  Einstein  theory  of  rela- 
tivity will  be  found  in  Chapter  XXIII. 


CHAPTER  XIII 

ECLIPSES  SINCE  1923 

WHAT  a  splendid  opportunity  seemed  to  await  the 
American  astronomer  at  the  eclipse  of  1923!     On 
September   10,  the  moon's  shadow  touched   the 
earth's  surface  at  sunrise  in  the  Pacific  off  the  coast  of  Japan. 
The  shadow  traversed  the  ocean  at  a  speed  well  over  a  thou- 
sand miles  per  hour  and  appeared  off  the  coast  of  southern 
California  somewhat  after  noon.    After  crossing  Lower  Cali- 
fornia, Mexico  and  Yucatan,  the  eclipse  ended  at  sunset  in 
the  Caribbean  Sea  north  of  British  Guiana. 

It  seemed  very  fortunate  that  the  total  eclipse  track  was 
to  pass  over  a  portion  of  California  where  one  naturally 
expects  during  the  summer  months  superb  conditions  of 
weather.  Everyone,  indeed,  has  heard  of  the  boasted  cli- 
mate of  southern  California,  the  "  land  of  sunshine  and  flow- 
ers." In  order  to  supplement  the  regular  observations  of 
the  U.  S.  Weather  Bureau,  the  Eclipse  Committee  of  the 
American  Astronomical  Society  for  four  years  previous  to 
1923  had  had  special  observations  made  of  the  wind  and 
weather  during  the  first  two  weeks  of  September  and  at  the 
hour  of  the  eclipse.  As  the  result  of  all  the  information  avail- 
able it  seemed  that  one  of  the  best  spots  for  an  eclipse  expedi- 
tion was  San  Diego,  the  only  large  city  in  the  United  States 
inside  the  path  of  totality.  Within  the  memory  of  the  "  old- 
est inhabitant "  there  had  not  been  a  single  cloudy  day  on 
the  tenth  of  September.  To  make  matters  almost  ideal,  the 
total  eclipse  came  at  one  o'clock  when  the  danger  from  sea- 
fog  was  reduced  to  a  minimum.  A  conservative  estimate 
placed  the  chances  of  perfect  conditions  at  least  ninety  per 
cent.  For  the  first  time  in  the  history  of  science,  the  astrono- 
mer was  able  to  insure  his  expedition  from  ill-luck  of  any 
kind,  from  clouds,  from  a  sudden  gust  of  wind  that  would 

203 


204  ECLIPSES    OF    THE    SUN 

shake  the  instruments  or  from  any  lack  of  adjustment  that 
would  endanger  the  final  perfection  of  the  photographs.  Al- 
though eclipse  insurance  was  offered  to  the  expedition  from 
the  Leander  McCormick  Observatory,  and  at  a  very  small 
premium,  it  was  not  accepted,  for  it  seemed  poor  business  to 
add  to  the  expenses  by  insuring  against  the  clouds  that 
weather  statistics  showed  to  be  so  improbable. 

The  author  had  made  arrangements  to  observe  his  fifth 
total  eclipse  of  the  sun  as  a  member  of  the  party  from  the 
United  States  Naval  Observatory  in  the  event  that  the  gov- 
ernment institution  should  send  its  own  expedition.  A  spe- 
cial eclipse  appropriation  not  having  been  passed  by  Congress 
and  a  Naval  Observatory  expedition  thus  being  impossible, 
the  necessary  spectroscopic  apparatus  was  loaned  to  the 
Leander  McCormick  Observatory.  To  add  to  his  kindness, 
the  superintendent  of  the  Naval  Observatory  personally  made 
arrangements  with  the  Secretary  of  the  Navy  and  the  Chief 
of  the  Bureau  of  Equipment  for  the  observers  and  equip- 
ment to  be  carried  by  naval  vessel  from  Hampton  Roads  on 
the  Atlantic  to  San  Diego. 

On  arrival  in  San  Diego  it  was  found  that  the  U.  S.  Army 
was  not  to  be  outdone  in  generosity  by  the  Navy.  Before 
leaving  the  East,  permission  had  been  secured  from  the 
Chief  of  Coast  Artillery  for  the  University  of  Virginia  party 
to  locate  within  the  military  reservation  of  Fort  Rosecrans. 
To  put  it  mildly,  the  writer  was  amazed  to  have  the  com- 
manding officer  of  Fort  Rosecrans  extend  an  invitation,  on 
behalf  of  the  government,  for  the  Virginia  party  to  take  up 
residence  in  officers'  quarters  in  the  Army  post.  Accord- 
ingly, six  weeks  before  the  eclipse,  the  McCormick  Observa- 
tory party  found  themselves  in  a  furnished  house  of  ten 
rooms  and  two  baths,  the  former  home  of  a  major.  Meals 
were  sent  in  from  the  company  kitchen  —  and  Rosecrans 
boasted  of  the  "  best  cook  in  the  Army  "! 

This  army  post  is  located  on  Point  Loma,  a  peninsula  lying 
between  the  Pacific  Ocean  to  the  west,  and  San  Diego  Bay 
on  the  east.  On  the  top  of  the  ridge  which  runs  out  to  the  old 
Spanish  lighthouse  is  a  famous  drive,  and  from  the  Point  it- 
self there  is  afforded  one  of  the  finest  views  in  the  world 


ECLIPSES    SINCE    1923  205 

overlooking  San  Diego  Bay  and  the  city,  North  Island  (with 
Rockwell  Flying  Field  and  Naval  Air  Station),  Coronado 
Beach  and  Tent  City;  to  the  south  lay  the  Coronado  Islands, 
belonging  to  Mexico;  to  the  east  and  southeast  the  moun- 
tains; to  the  west  the  blue  water  of  the  Pacific. 

San  Diego  has  every  reason  to  be  proud  of  her  city,  located 
as  it  is  in  one  of  the  finest  spots  on  the  globe  and  blessed  with 
an  agreeable  climate  that  might  well  be  the  envy  of  any 
city  in  the  world.  We  from  Virginia,  a  state  which  has  never 
been  backward  in  painting  a  halo  around  everything  con- 
nected with  the  Commonwealth,  were  much  interested  in  the 
spirit  of  civic  pride  and  aggressiveness,  which  at  times  may 
have  even  bordered  upon  boastfulness.  Our  six  weeks'  stay 
was  filled  with  pleasant  memories  and  we  only  wish  that  each 
and  every  total  eclipse  might  be  observed  under  the  con- 
genial surroundings  of  Fort  Rosecrans. 

Many  are  the  problems  that  may  be  profitably  attacked 
at  the  time  of  a  total  eclipse.  One  of  the  most  important 
at  the  1923  eclipse  was  the  confirmation  of  the  bending  of 
the  rays  of  light  from  a  star  as  these  rays  pass  close  to  the 
edge  of  the  sun,  as  predicted  by  Einstein.  We  had  no  equip- 
ment for  this  research  and  we  preferred  to  let  others  tackle 
this  problem  while  we  devoted  ourselves  to  following  up  the 
line  of  attack  carried  out  at  four  previous  eclipses.  It  was 
therefore  decided  to  have  two  eclipse  stations,  each  equipped 
with  a  powerful  spectrograph.  One  of  these  was  located  on 
Point  Loma,  the  other  at  Lakeside,  twenty-five  miles  inland, 
and  near  the  edge  of  the  shadow  cast  by  the  moon.  The 
essential  part  of  each  spectrograph  was  a  concave  grating 
ruled  by  Rowland  of  Johns  Hopkins.  Each  of  the  gratings 
used  was  made  by  ruling  on  a  spherical  concave  mirror  of 
speculum  metal  15,000  lines  to  each  inch. 

At  North  Island  was  the  powerful  battle  squadron  of  the 
U.  S.  Naval  Aircraft  forces.  Here  was  a  chance  to  employ  pho- 
tography from  the  air  on  any  of  the  problems  that  could  be 
solved  by  this  method.  It  is  manifestly  impossible  to  use  any 
but  comparatively  small  cameras  from  an  airplane  and  to  give 
but  very  brief  exposures.  On  account  of  the  short  exposures 
permitted  no  spectroscopic  work  could  be  attempted  from  the 


206  ECLIPSES    OF    THE    SUN 

air,  no  investigation  of  the  Einstein  effect  and  no  photog- 
raphy of  the  corona  that  demanded  large  focal  scale.  Air- 
plane photographs  could  not  compete  with  those  taken  from 
a  fixed  installation  on  terra  firma.  In  the  event  of  clouds 
and  the  possibility  of  soaring  above  them  in  a  machine,  air- 
plane photographs  might  be  taken,  but  there  would  be  little 
of  scientific  value  in  photographing  the  corona  on  such  a 
small  scale.  There  seemed  only  one  direction  in  which 
photography  from  the  air  could  assist  the  astronomer,  and 
that  was  in  the  attempt  to  find  the  position  of  the  moon  in 
the  sky  with  greater  accuracy. 

The  program  finally  adopted  consisted  in  attempting,  from 
five  separate  stations  along  the  northern  edge  and  one  at  the 
southern  edge  on  the  shore  of  Mexico,  to  photograph  from 
the  air  the  ground  underneath  intersected  by  the  edge  of 
the  shadow  of  the  moon.  For  this  purpose  it  was  necessary 
to  use  the  best  of  mapping  cameras  known  to  the  photog- 
raphers and  to  choose  special  sites  to  photograph  where  the 
terrain  would  offer  as  great  contrast  as  possible  between  a 
point  just  inside  and  one  just  outside  the  moon's  shadow.  In- 
cidentally it  must  be  admitted  that  so  little  was  known  of  the 
amount  of  light  to  be  expected  a  few  yards  outside  of  the 
moon's  shadow  that  there  was  some  doubt  as  to  the  final  suc- 
cess of  the  investigation.  But  there  was  nothing  to  do  but 
to  "  try  and  see  what  happened."  To  supplement  this  work 
from  the  air,  there  were  two  parties  of  sailors,  each  of  seventy- 
five,  located  at  the  expected  edge  of  the  moon's  shadow.  The 
sailors  were  put  at  intervals  of  ten  feet  at  right  angles  to  the 
edge  of  the  shadow,  and  each  was  instructed  to  note  whether 
he  could  see  the  corona  or  not.  If  he  could  see  the  corona 
during  totality,  he  was  presumably  inside  the  penumbra  of 
the  moon's  shadow. 

In  addition  to  the  pilots  with  mapping  cameras,  there  were 
others  with  movie  cameras  stationed  over  Point  Loma  to  pho- 
tograph the  coming  of  the  eclipse,  each  provided  with  a  navy 
chronometer-watch  to  detect  the  accurate  time,  while  an- 
other plane  went  aloft  equipped  with  self-recording  appara- 
tus to  register  temperature,  humidity,  etc.,  during  the  total 
phase. 


ECLIPSES    SINCE    1923  207 

During  the  erection  of  our  apparatus  it  was  interesting  to 
watch  the  gradual  installation  of  the  gigantic  equipment  to 
be  used  on  eclipse  day  near  the  tip  of  Point  Loma  by  the  as- 
tronomers from  the  Mt.  Wilson  Observatory.  This  great 
observatory,  the  best  equipped  and  most  famous  in  the  whole 
world,  is  located  on  Mt.  Wilson  near  Pasadena.  Being  there- 
fore only  about  140  miles  from  the  eclipse  site,  it  was  possible 
to  transport  all  of  their  instruments  by  motor  truck  from 
observatory  shops  to  eclipse  camp.  The  problem  of  trans- 
porting the  heavy  apparatus  was  a  very  simple  one  for 
the  Mt.  Wilson  staff,  and  one  with  which  they  are  very 
familiar. 

To  continue  the  magnificent  work  begun  by  Michelson  in 
measuring  the  angular  diameter  of  the  star  Betelgeuse,  it  was 
found  to  be  necessary  to  separate  the  plane  mirrors  of  the 
interferometer  to  the  great  distance  of  fifty  feet.  As  this 
meant  too  large  a  span  to  be  attached  to  the  loo-inch  reflector, 
a  separate  mounting  was  devised.  This  consisted  in  a  bridge- 
like  truss  attached  to  a  heavy  polar  axis  driven  by  clock 
mechanism.  The  clock  and  central  section  of  the  mounting 
having  been  completed,  the  director  of  the  Mt.  Wilson  Ob- 
servatory decided  to  utilize  this  interferometer  structure  on 
which  to  mount  cameras,  spectroscopes,  photometers,  etc.,  for 
the  eclipse  program.  Here  was  a  startling  innovation  in 
eclipse  work,  to  put  all  of  the  instruments  on  one  mounting, 
"  the  eggs  all  in  one  basket " —  but  the  scientific  world  has 
learned  to  have  confidence  in  the  judgment  of  the  Mt.  Wil- 
son astronomers.  Their  longest  camera  was  thirty  feet  in 
focal  length,  used  to  photograph  the  Einstein  effect.  An- 
other camera  half  this  length  was  for  the  same  purpose. 
There  were  cameras  to  portray  the  beauties  of  the  corona  in 
various  scales  and  in  different  colors  of  light,  spectroscopes 
to  ascertain  the  constitution  of  the  corona,  instruments  to 
photograph  and  to  measure  visually  the  intensity  of  the  light 
of  the  corona  at  various  angles  from  the  edge  of  the  sun.  The 
instruments  (see  page  208)  numbering  fifteen  made  prob- 
ably the  most  complete  equipment  that  had  ever  been  assem- 
bled for  photographing  a  solar  eclipse.  The  personnel  in- 
cluded about  thirty  members  of  the  Mt.  Wilson  staff  while  an 


208  ECLIPSES    OF    THE    SUN 

auxiliary  party  of  twenty  computers  and  friends  of  the  staff 
were  prepared  to  watch  and  measure  shadow  bands,  etc. 

To  the  astronomers  gathered  for  the  eclipse  the  weather 
seemed  "  unusual  "  (we  understand  this  is  the  first  word 
that  a  California  baby  learns  at  its  mother's  knee).  At  any 
rate  the  cloudless  skies  that  we  had  been  led  to  expect  at 
1:00  P.M.  were  not  always  forthcoming.  During  the  first 
two  weeks  of  our  stay  at  Point  Loma,  high  fog  at  noon  was 
the  rule  rather  than  the  exception.  Still  it  was  a  long  time 
to  the  eclipse,  and  conditions  would  undoubtedly  improve 
—  and  they  did.  The  next  two  weeks  gave  perfect  skies,  an 
absence  of  wind  and  altogether  ideal  conditions.  Would 
this  last,  or  would  another  cloudy  spell  come?  We  were 
optimists  and  believed  implicitly  what  our  friends  the  Cali- 
fornians  told  us. 

Saturday,  September  8,  was  a  cloudy  day  and  the  eclipse 
would  have  been  under  poor  conditions.  Sunday  was  even 
worse.  What  would  Monday  bring?  Lieutenant  Wyatt  in 
charge  of  the  weather  observations  at  North  Island  made 
daily  predictions  of  weather  conditions  to  serve  as  a  guide 
for  airplane  flights.  He  telephoned  me  twice  on  Sunday.  His 
report  was,  "  There  has  been  excessively  hot  weather  in  Im- 
perial Valley  for  the  past  week.  A  secondary  low  over  North- 
ern California  makes  the  prospect  of  clear  weather  tomorrow 
very  doubtful.  If  the  back  of  the  hot  wave  is  broken  there 
may  be  a  change.  This  will  be  signalled  by  a  brisk  wind  from 
the  west.  Unless  this  wind  springs  up  in  the  night,  I  am 
afraid  for  you  tomorrow. " 

At  eight-thirty  eclipse  morning  we  were  off  on  our  trip  to 
Lakeside.  As  we  got  inland  from  the  Pacific,  out  of  reach 
of  the  high  fog,  and  where  ordinarily  we  got  out  of  the 
clouds,  the  conditions  improved  but  little.  At  ten  o'clock 
at  Lakeside  we  found  high  cirrus  and  the  conditions  looked 
hopeless.  But  there  were  three  good  hours  until  the  time 
of  totality  and  much  to  be  done  in  the  final  preparations! 
By  twelve  o'clock  we  heard  the  airplane  aloft  which  was  to 
observe  over  Lakeside  and  the  pilot  waved  to  us  in  friendly 
greeting.  The  clouds  were  not  very  heavy  but  there  were 
no  clear  patches  anywhere.  We  kept  a  stiff  upper  lip  and 


MT.  WILSON  INSTRUMENTS  BEING  ASSEMBLED  FOR  THE  ECLIPSE  OF  1923 


ECLIPSES    SINCE    1923  209 

refused  to  believe  that  after  so  much  bragging  California 
was  going  to  get  a  black  eye. 

Still  the  clouds  were  not  too  thick  to  prevent  us  watching 
the  diminishing  crescent  of  the  sun  as  totality  approached. 
Nature  was  hushed  but  the  cocks  were  crowing  lustily  as  if 
night  were  falling.  At  the  very  second  when  expected  from 
the  revised  times  sent  out  from  the  American  Ephemeris, 
the  sun  was  blotted  out  and  a  faint  trace  of  corona  appeared. 
But  what  a  bitter  disappointment !  We  carried  through  our 
program,  exposing  eight  plates  in  the  seventy  seconds  of 
totality,  knowing  full  well  that  the  developed  plates  would 
show  not  the  slightest  trace  of  light. 

Well,  there  was  no  use  crying  over  it  (although  one  was 
tempted  to).  We  had  done  our  best  and  the  fault  was  not 
ours.  If  misery  likes  company,  it  was  evident  that  the  clouds 
were  general  and  not  local.  As  quickly  as  a  high-powered 
car  could  take  us  to  San  Diego  we  telephoned  to  Point  Loma 
only  to  find  as  we  expected,  that  conditions  there  were  even 
worse  and  practically  not  a  trace  had  been  seen  of  the  corona. 
Radio  soon  told  us  that  the  large  assemblage  of  seventy  as- 
tronomers on  Catalina  Island,  including  the  Yerkes  *  and 
Harvard  parties,  Plaskett,  Stebbins,  Fox,  Wilson  and  others, 
had  suffered  a  like  fate.  Even  the  usual  luck  of  the  Lick 
party  had  deserted  them,  for  conditions  where  Dr.  Campbell 
and  the  Lick  expedition  were  located  at  Ensenada  2  were 
about  as  bad  as  could  possibly  be. 

And  this  was  the  record  for  "  sunny  California  "!  Not  a 
single  expedition  greeted  with  good  conditions,  and  the  whole 
scientific  work  a  dismal  failure!  There  was  nothing  left  to 
do  but  pack  up  and  go  home  —  and  then  begin  to  get  ready 
for  the  next  eclipse. 

The  weather  conditions  in  Mexico  did  not  give  promise 
of  the  superb  conditions  that  had  been  expected  in  southern 
California,  so  it  was  all  the  more  gratifying  to  find  the  good 
luck  experienced  by  the  astronomers  in  old  Mexico.  Four 
separate  expeditions  were  wholly  or  partially  successful.  The 
Mexican  government  financed  two  expeditions,  one  from  the 

1  Popular  Astronomy,  32,  205,  1924. 
-  Publications  A.  S.  P.,  35,  275,  1923. 


210  ECLIPSES    OF    THE    SUN 

Mexican  National  Observatory  of  Tacubaya  under  the  di- 
rection of  Gallo,  the  other  L  consisting  of  Schorr,  Luden- 
dorff  and  Kolhschiitter  from  Germany.  The  Steward  Ob- 
servatory of  Arizona  had  an  expedition 2  in  the  province  of 
Sonora  and  the  Sproul  party  3  was  located  at  Yerbanis.  Di- 
rector J.  A.  Miller  had  the  capable  assistance  of  Heber  D. 
Curtis,  director  of  the  Allegheny  Observatory. 

The  Sproul  expedition  had  a  varied  program  of  work,  but 
mention  will  be  made  here  of  two  items  only,  photographs 
with  Einstein  cameras  of  fifteen  feet  focus  and  photographs 
on  a  larger  scale  of  sixty-five  feet  focus,  the  cameras  in  each 
case  being  pointed  directly  at  the  sun.  The  photographs  for 
the  Einstein  effect,  one  of  which  is  reproduced  facing  page 
212,  showed  the  corona  in  splendid  detail.  The  star  images 
however  were  not  of  the  best  definition,  and  as  the  photo- 
graphs were  made  through  thin  haze,  it  was  decided  by  Miller 
that  he  would  not  measure  the  plates  to  obtain  the  deflec- 
tions of  the  star  images. 

After  the  ill  luck  experienced  by  the  American  astronomers 
in  1923  where  conditions  beforehand  had  seemed  so  promis- 
ing, it  seemed  almost  foolhardy  to  prepare  for  an  eclipse 
which  was  to  take  place  at  nine  o'clock  on  a  winter's  morning 
on  the  Atlantic  coast  in  the  United  States  with  the  sun  at  best 
only  eighteen  degrees  above  the  horizon.  Would  snow  cover 
up  the  astronomers  as  had  happened  on  January  14,  1907, 
the  preceding  eclipse  in  this  cycle  of  the  Saros?  The  most 
optimistic  placed  the  chances  of  good  weather  about  fifty 
per  cent.  With  such  poor  prospects  it  was  natural  that  none 
but  American  astronomers  would  plan  to  observe  the  eclipse. 
In  fact,  the  chances  of  success  seemed  so  remote  that  the 
Lick  Observatory,  with  its  splendid  record  of  carefully 
planned  observations  secured  at  many  eclipses,  decided 
against  sending  an  expedition  across  the  continent.  After 
attempting  a  very  large  program  at  the  1923  eclipse,  the 
Mount  Wilson  Observatory  contented  itself  in  1925  with  at- 
tacking about  one-fifth  of  the  problems  planned  for  the 
previous  eclipse. 

1  Sitz.  Preuss.  Akad.,  83,  1925. 

2  Publications  A.  S.  P.,  36,  170,  1924. 

3  Astroph.  Jour.,  61,  73,  1925;  Sproul  Obs.  Publ.,  No.  7, 


ECLIPSES    SINCE    1923  211 

What  a  surprise  the  weather  again  had  in  store  for  us  on 
January  24,  1925!  The  day  before  the  eclipse  was  one  of 
gorgeously  perfect  blue  skies.  Would  the  morrow  provide 
equally  good  skies?  The  clear  skies  continued  throughout 
the  night  but  it  clouded  over  completely  at  six  in  the  morn- 
ing—  and  totality  occurred  three  hours  later.  The  largest 
group  of  astronomers  was  assembled  at  Middletown,  Con- 
necticut, at  the  Van  Vleck  Observatory,  and  here  were  parties 
representing  the  Mount  Wilson  Observatory,  Leander  Mc- 
Cormick  Observatory,  Harvard  University,  Universities  of 
Wisconsin  and  Illinois,  United  States  Bureau  of  Standards, 
and  many  others.  What  a  dejected  crowd  we  were  at  eight 
o'clock  when  we  had  gathered  at  the  Van  Vleck  Observatory 
to  observe  first  contact,  the  beginning  of  the  eclipse.  There 
was  nothing  but  clouds  everywhere!  A  quarter  of  an  hour 
later  a  ray  of  hope  appeared;  there  was  a  blue  streak  of  sky 
low  down  in  the  northwest  —  and  the  clouds  were  coming 
from  that  quarter.  Would  it  clear  off  in  time?  Luck  was 
with  us.  Five  minutes  before  totality  a  cloud,  very  thin  and 
very  fleecy,  hung  over  the  sun.  It  was  not  thick  enough  to 
do  much  damage  and  it  was  moving  slowly.  We  hoped  it, 
too,  would  go.  When  the  timers  called  out  "  two  minutes," 
the  cloud  was  almost  gone.  Now  it  was  beginning  to  get 
quite  dark,  a  weird  and  unnatural  pall  coming  over  the  land- 
scape. The  observers  outside  noticed  shadow  bands  flicker- 
ing over  the  snow.  At  one  minute  before  totality,  with  the 
thin  crescent  of  the  sun  growing  very  small,  the  atmospheric 
conditions  seemed  perfect,  the  thin  cloud  had  gone! 

How  fortunate  we  were  that  the  weather  defied  all  the 
laws  of  averages  on  January  24,  1925!  With  clouds  hover- 
ing everywhere  over  New  York  and  New  England,  the  sur- 
prising fact  was  that  clear  skies  greeted  most  of  the  as- 
tronomical expeditions.  It  was  cloudy  throughout  Michigan 
and  Ontario,  cloudy  in  Buffalo,  but  clear  at  Ithaca,  Pough- 
keepsie,  Middletown,  New  Haven,  Nantucket  and  New  York. 
It  is  estimated  that  ten  million  people  in  New  York  State  and 
New  England  were  given  an  opportunity  of  witnessing  the 
gorgeous  spectacle.  The  great  newspapers  of  New  York  City 
asserted  that  no  single  event  in  the  past  decade  has  aroused 


212  ECLIPSES    OF    THE    SUN 

such  widespread  enthusiasm.  As  totality  lasted  for  the  brief 
time  of  two  minutes  or  less,  and  as  the  scientific  investiga- 
tions were  nearly  all  crowded  within  the  period  of  the  total 
phase  of  the  eclipse,  it  is  probable  that  no  single  event  in  the 
history  of  man  has  had  so  many  words,  per  minute  of  the  dura- 
tion of  the  event,  written  about  it  as  has  the  1925  eclipse. 
As  a  spectacle  this  eclipse  suffered  from  taking  place  so  early 
in  the  morning.  If  the  darkening  had  come  on  during  the 
middle  of  the  day  with  the  sun  high  up  in  the  sky,  the  psy- 
chological effect  would  have  been  greater.  The  shape  of  the 
corona  corresponded  closely  with  that  expected  from  the 
condition  of  the  sun  near  spot  minimum.  To  the  right  of 
the  vertical,  however,  there  was  a  long  pointed  shaft  of  light 
stretching  up  more  than  a  degree.  The  remarkable  feature 
was  the  total  lack  of  rosy  color  visible  to  the  naked  eye,  no 
prominences  being  readily  visible,  and  as  a  result  the  corona 
lacked  color  as  if,  possibly,  to  reflect  the  feelings  of  the  ob- 
servers who  everywhere  worked  with  the  thermometer  below 
the  zero  mark  of  the  Fahrenheit  scale. 

Several  unusual  features  concerning  this  eclipse  are  worthy 
of  note.  The  eclipse  track  crossed  over  the  Van  Vleck  Ob- 
servatory with  its  visual  refractor  of  20  inches  aperture. 
Here  was  an  opportunity  of  photographing  the  corona  with  a 
large  telescope  and  with  yellow  light.  Miller  of  the  Sproul 
Observatory  used  the  photographic  lens  of  63 -feet  focal 
length,  mounted  as  in  1918  and  1923,  pointed  directly  at  the 
sun.  Naturally  there  were  hosts  of  cameras  of  smaller  focal 
length.  On  account  of  the  high  possibility  of  clouds,  the 
United  States  Navy  employed  the  dirigible  Los  Angeles  which 
flew  out  to  sea  and  carried  a  battery  of  cameras  with  which 
to  photograph  the  corona.  Even  in  a  gigantic  dirigible  the 
platform  carrying  the  cameras  is  not  very  steady  and  it  is 
very  difficult  to  keep  the  cameras  pointed  at  the  sun.  Natu- 
rally it  was  possible  to  utilize  the  cameras  only  of  short  focal 
length  and  to  give  brief  exposures  only.  If  it  had  been 
known  beforehand  that  the  skies  were  to  be  clear  at  eclipse 
time,  the  Los  Angeles  would  not  have  taken  part  in  the 
eclipse  program.  Its  photographs  showed  the  general  fea- 
tures of  the  corona  but  they  had  little  of  scientific  value  com- 


CORONA  OF  SEPTEMBER  10,  1923 
Sproul  Observatory  photograph  with  15 -foot  camera. 


fl 
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ECLIPSES    SINCE    1923  213 

pared  with  those  taken  by  cameras  with  more  stable  founda- 
tions and  greater  focal  lengths.  Many  airplanes  were  utilized 
for  the  same  mission. 

Many  attempts  were  made  to  measure  the  intensity  of  the 
corona,  among  which  should  be  noted  the  work  of  Nicholson 
and  Pettit,  by  Stebbins  and  King,  and  by  Coblentz  and  Stet- 
son, all  three  parties  gathered  in  Middletown;  and  also  by 
Parkhurst,  located  farther  west  in  Ithaca.  Curtis  at  New 
Haven,  Anderson  and  Mitchell,  both  in  Middletown,  photo- 
graphed the  flash  spectrum,  the  first  named  paying  special 
attention  to  the  red  end  and  reaching  to  wave-length  8800  A 
on  the  photographs.  The  results  obtained  by  these  various 
observers  will  be  duly  recorded  on  the  appropriate  pages 
later  in  this  book. 

The  developed  photographs  all  show  the  effect  of  the  low 
altitude  of  the  sun  and  the  consequent  poor  seeing.  As 
eclipses  are  of  such  rare  occurrence  it  is  fortunate  that  coronal 
details  do  not  require  the  finest  conditions  of  seeing  for  their 
precise  portrayal.  The  details  of  the  coronal  streamers  are 
not  inherently  sharp  and  clear-cut  in  their  nature,  and  conse- 
quently a  little  blurring  of  detail  caused  by  poor  atmospheric 
conditions  has  little  deleterious  effect  on  the  photographs. 
The  case  is,  however,  very  different  with  the  prominences. 
By  nature  these  phenomena  have  more  definite  outlines.  The 
best  photographs  with  which  to  test  the  quality  of  the  seeing 
at  the  1925  eclipse  were  unquestionably  those  taken  with 
the  20-inch  refractor  of  the  Van  Vleck  Observatory.  This 
telescope  is  regularly  employed  in  the  determination  of  stellar 
parallaxes  bv  photography.  The  focus  consequently  has 
been  thoroughly  well  determined.  The  plates  taken  by  this 
telescope  (which  has  the  largest  aperture  ever  used  to  pho- 
tograph an  eclipse)  were  a  great  disappointment,  due  mainly 
to  the  lack  of  sharp  detail  shown  in  the  inner  corona. 
The  fault  did  not  lie  in  the  telescope  or  in  any  lack  of  care- 
ful adjustment,  but  was  to  be  found  in  causes  beyond  the 
control  of  the  observer,  namely,  the  poor  definition  resulting 
from  bad  seeing.  The  same  qualities  of  poor  seeing  are  found 
in  the  spectral  images  of  these  prominences  in  the  flash  spec- 
trum taken  without  slit. 


214  ECLIPSES    OF    THE    SUN 

At  the  eclipse  of  1923,  the  preparations  made  to  photo- 
graph from  airplanes,  so  as  to  fix  with  greater  exactness  the 
place  of  the  moon  in  the  sky,  came  to  naught  because  of 
clouds.  In  1925,  on  account  of  the  southern  edge  of  the 
moon's  shadow  passing  over  New  York  City,  an  excellent 
opportunity  was  at  hand  to  secure  many  observers  to  deter- 
mine the  exact  edge  of  the  shadow. 

Calculations  based  on  the  American  Ephemeris  had  fore- 
told that  the  edge  of  the  moon's  path  would  cut  Riverside 
Drive  (which  runs  north  and  south  along  the  Hudson  River) 
somewhere  between  83d  Street  and  noth  Street,  with  a 
total  uncertainty  of  approximately  one  mile.    To  make  cer- 
tain that  the  astronomers  were  not  mistaken,  observers  were 
located  at  each  intersection  of  city  blocks  all  the  way  from 
72d  Street  to  i3Sth  Street,  usually  on  the  tops  of  apartment 
houses,  so  that  a  better  view  might  be  obtained.    Sixty-nine 
men  were  employed  and  each  was  furnished  with  a  piece  of 
darkened  glass  and  was  instructed  to  look  at  the  sun  at  the 
time  of  totality  in  order  to  see  whether  the  corona  was  visible 
or  whether  there  was  a  thin  edge  of  the  sun  left  shining.    All 
of  the  observers  were  instructed  to  report  to  a  central  office 
immediately  after  the  eclipse.    Only  one  of  the  total  of  sixty- 
nine  was  in  doubt  as  to  what  he  saw,  and  the  sixty-eight  gave 
a  clear-cut  verdict.    The  observer  at  240  Riverside  Drive 
had  seen  the  total  eclipse  while  a  man  located  at  230  River- 
side Drive  had  seen  a  small  sliver  of  the  sun  exposed,  indi- 
cating that  it  was  a  partial  eclipse.    The  distance  between 
these  two  men  was  about  two  hundred  and  twenty-five  feet, 
which  included  the  width  of  9 6th  Street.    The  edge  of  the 
shadow  of  totality  was,  therefore,  pinned  down  to  within 
two  hundred  and  twenty-five  feet  on  the  west  edge  of  Man- 
hattan Island.    Other  observers  along  the  East  River  were 
successful  in  making  similar  observations  with  the  result 
that  the  moon's  path  across  New  York  City  is  accurately 
known.     It  is  indeed  surprising  to  find  such  unanimity  of 
opinion  among  untrained  observers  who  were  all  witnessing 
their  first  eclipse.    Apparently  it  must  be  very  easy  to  make 
up  one's  mind  as  to  whether  the  corona  is  or  is  not  visible. 
Similar  attempts  have  been  made  at  former  eclipses  but  have 


ECLIPSES    SINCE    1923  215 

always  met  with  failure.  None  of  the  observers  saw  the 
edge  of  the  moon's  shadow  as  it  lay  upon  the  ground  in  spite 
of  the  excellent  opportunity  afforded  on  account  of  the  ground 
being  completely  covered  with  snow.  Evidently  the  edge 
of  the  shadow  is  not  sharply  defined  but  the  light  tapers  off 
gradually. 

The  keen  interest  of  astronomers  in  total  eclipses  of  the 
sun  again  manifested  itself  in  the  Dutch  East  Indies  on 
January  14,  1926.  No  less  than  nine  expeditions  from 
Europe  and  America  were  gathered  for  the  purpose  of  work- 
ing ardently  and  enthusiastically  for  the  brief  period  of  four 
minutes.  Three  expeditions,  numbering  twenty  people,  trav- 
eled from  the  United  States  halfway  round  the  globe  in  the 
hope  of  having  clear  skies  during  the  few  precious  minutes 
of  totality. 

In  recounting  the  history  of  eclipses,  repeated  attention 
has  been  called  to  the  many  failures  of  eclipse  expeditions 
due  to  the  fickle  weather.  This  has  been  especially  true  in 
the  tropics.  Where  the  writer  was  located  in  Sumatra  for 
the  1901  eclipse  the  annual  rainfall  was  the  goodly  amount 
of  1 86  inches  or  an  average  of  half  an  inch  per  day. 

Unfortunately,  at  the  1926  eclipse  the  weather  lived  up  to 
its  poor  reputation.  Clouds  were  the  rule  rather  than  the 
exception.  The  British  party  at  Benkoelen  on  the  west 
coast  of  Sumatra  had  the  best  luck.  Their  program1  was 
almost  exclusively  spectroscopic.  Excellent  spectra  were 
obtained  which  will  be  described  in  a  subsequent  chapter. 

Although  the  party  from  Swarthmore 2  was  located  not  far 
from  the  British  in  Benkoelen,  they  did  not  fare  so  well  with 
the  weather.  Successful  photographs  were  secured  with  the 
large  camera  of  63 -feet  focal  length  pointed  directly  at  the 
sun,  with  twin  cameras  of  i5-feet  focus  for  testing  the  Ein- 
stein deflection,  and  with  smaller  cameras.  Curtis  again  used 
the  short  focus  concave  grating  for  the  flash  spectrum  but  on 
account  of  the  haze  and  the  deterioration  of  the  plates 
stained  with  dicyanine  no  spectra  were  obtained. 

The  Harvard  expedition,  consisting  of  Stetson,  Coblentz 

1  Memoirs  R.  A.  5.,  64,  105,  1927. 

2  Popular  Astronomy,  34,  349,  1926. 


216  ECLIPSES    OF    THE    SUN 

and  Arnold,  measured  the  intensity  of  coronal  radiation  visu- 
ally and  photographically.  Their  results  seemed  to  show 
that  the  total  coronal  light  was  40  per  cent  greater  in  1926 
than  in  1925.  However,  their  work  was  done  at  the  site 
of  the  Swarthmore  party  and  it  is  possible  that  the  haze  may 
have  affected  their  results.  (See  Chapter  XXI.) 

On  the  other  side  of  the  mountain  range  from  Benkoelen 
was  the  expedition  from  the  U.  S.  Naval  Observatory  includ- 
ing Anderson  of  Mount  Wilson  with  a  21 -foot  concave  grat- 
ing for  the  flash  spectrum.  The  eclipse  was  practically  lost 
through  clouds  covering  the  sun  for  nearly  the  whole  of 
totality.  Freundlich  of  Potsdam  located  at  Benkoelen  on 
account  of  haze  secured  few  photographs  of  value.  In  East 
Africa,  Horn  d'Arturo  l  at  the  head  of  a  completely  equipped 
Italian  expedition  was  more  successful  with  the  weather. 
The  photographs  of  the  corona  taken  by  him  were  compared 
with  those  obtained  in  Sumatra.  Many  changes  were  noted 
giving  evidence  of  motions  in  the  coronal  structure,  particu- 
larly in  the  domes  which  are  such  interesting  phenomena  at 
each  eclipse. 

On  June  29,  1927,  the  eclipse  track  passed  over  England, 
Norway,  Northern  Sweden,  the  Arctic  Ocean  and  North- 
eastern Siberia.  Unfortunately  the  eclipse  was  a  very  brief 
one.  At  Liverpool,  totality  took  place  at  5:24  A.M.  and 
lasted  only  twenty-three  seconds.  At  Fagernes,  Norway, 
totality  occurred  at  5:34  (Greenwich  time)  and  had  a  dura- 
tion of  34  seconds.  To  make  matters  worse,  the  probability 
of  clear  weather  was  not  very  great.  In  England,  at  the 
early  hour  of  the  morning  that  the  eclipse  took  place,  the 
most  optimistic  estimated  that  the  chance  of  clear  weather 
was  no  more  than  one  out  of  three.  In  Norway,  away  from 
the  coast  and  farther  along  the  eclipse  track  in  Lapland,  the 
chances  of  clear  weather  appeared  to  be  greater  but  at  best 
were  no  more  than  an  even  chance.  It  seemed  almost  fool- 
hardy to  attempt  to  observe  this  eclipse  with  such  a  short 
duration  of  totality  and  with  the  chances  of  clear  weather 
reduced  to  the  minimum.  Still  nothing  venture,  nothing 
erain!  It  was  certain  beforehand  that  no  great  astronomical 

1  Publ.  d.  Oss.  Astron.  di  Bologna,  i,  227,  1926. 


ECLIPSES    SINCE    1923  217 

discoveries  would  be  made  from  coronal  investigations  with 
this  short  duration.  Still  it  was  very  important  that  a  record 
should  be  made  of  the  eclipse.  For  photographing  the 
flash  spectrum  the  short  duration  of  totality  was  no  great 
drawback. 

The  popular  enthusiasm  in  England  was  unbounded.  Eng- 
land had  not  witnessed  a  total  solar  eclipse  for  more  than 
two  centuries  (see  page  58)  and  the  populace  was  deter- 
mined to  attempt  to  see  the  wonderful  phenomenon  no  mat- 
ter how  dismal  were  the  chances  of  success.  Nearly  all  of 
the  British  astronomers  were  in  the  eclipse  track  in  England 
with  well-planned  expeditions.  A  large  and  well-equipped 
party  from  the  Solar  Physics  Observatory  of  Cambridge, 
however,  forsook  England  for  the  greater  promise  of  clear 
skies  in  Norway  and  set  up  their  equipment  at  Aal  under 
the  direction  of  H.  F.  Newall.  The  only  large  expedition 
from  the  United  States  was  located  in  Norway  at  Fagernes, 
fifty  miles  across  country  from  the  Cambridge  party.  The 
McCormick-Chaloner  party  from  the  University  of  Virginia 
had  a  very  complete  equipment  to  photograph  the  flash  spec- 
trum with  three  concave  gratings,  each  used  without  slit  in 
the  attempt  to  secure  the  flash  r  spectrum  from  the  extreme 
ultra-violet  to  the  far  red,  the  plates  for  the  region  of  long 
wave  lengths  being  stained  with  neocyanine.  Stetson  was 
a  member  of  the  party  at  Fagernes  with  the  instrumental 
equipment  he  had  used  at  the  eclipses  of  1925  and  1926  for 
measuring  the  radiation  of  the  corona.  The  astronomers 
from  Oslo  were  also  at  Fagernes.  Rosseland  had  a  21 -foot 
concave  grating  for  photographing  the  flash  spectrum,  Lohse 
attempted  to  observe  the  times  of  contacts  visually  and  by 
photography,  Stormer  had  a  number  of  small  cameras  for 
photographing  the  corona.  A  scientific  party  of  about  forty, 
of  ten  different  nationalities,  were  assembled  at  Fagernes. 
Other  parties  were  located  farther  along  the  eclipse  track, 
mainly  at  Ringebu  in  Norway,  and  at  Gallivare  and  Jokk- 
mokk  in  Lapland. 

Both  in  England  and  in  Scandinavia  the  weather  before 
the  eclipse  was  much  worse  than  had  been  expected.  At 
Fagernes,  instead  of  being  clear  for  half  the  time,  it  had 


218  ECLIPSES    OF    THE    SUN 

been   cloudy   for    fourteen   days   before   eclipse   day   and 
rain  had  fallen  on  nine  of  these  days.     Decided  optimism 
was  required  in  order  to  expect  a  sudden  break  in  the  weather 
conditions.    In  England  in  addition  to  the  cloudy  weather, 
there  were  very  high  winds  that  threatened  to  carry  away 
the  temporary  installations.    At  this  date  of  the  year,  June 
29,  and  at  the  high  northern  latitudes,  the  adjustment  of  the 
eclipse  instruments  had  to  be  accomplished  without  the 
stars.     At  Fagernes  (latitude  61°)  it  did  not  get  dark  at 
night,  while  Jokkmokk  was  in  the  land  of  the  midnight  sun. 
Eclipse  day  dawned  everywhere  with  the  weather  a  con- 
tinuation of  the  cloudy  conditions  of  the  previous  fortnight. 
Apparently  only  a  miracle  could  save  the  situation  and  bring 
clear  skies.    The  miracle  actually  did  take  place  in  England 
at  Giggleswick,  where  was  located  the  party  from  the  Green- 
wich Observatory  under  the  direction  of  the  Astronomer 
Royal,1  Sir  F.  W.  Dyson.    Here  the  sky  was  cloudy  through- 
out the  whole  of  eclipse  day.    However  a  hole  appeared  in 
the  cloudy  sky  and  lasted  for  a  space  of  two  minutes  only. 
The  eclipsed  sun  appeared  in  this  blue  patch  of  sky  and  suc- 
cessful photographs  were  secured.     The  photographs  show 
exquisite  detail  in  the  inner  corona  but  the  outer  corona  was 
lost  through  thin  haze.     At  Jokkmokk  where  was  located 
the  expedition  from  Hamburg  under  the  direction  of  Schorr, 
a  similar  miracle  took  place,  the  sun  at  totality  being  sur- 
rounded on  all  sides  by  clouds.    Schorr  was  more  successful 
than  Dyson,  the  sky  being  without  haze.    The  photographs 
taken  at  Giggleswick  and  Jokkmokk  exhibit  many  brilliant 
prominences  with  the  hooded  forms  overtopping  the  promi- 
nences showing  as  conspicuous  features.   A  comparison  of  the 
two  photographs  shows  well-marked  changes  taking  place 
during  the  time  elapsed.     Schorr's  photographs  show  the 
corona  to  be  of  the  circular  type  that  was  expected  at  the  time 
near  maximum  of  sun-spots.    The  brilliant  prominences  and 
extended  corona  must  have  made  a  gorgeous  spectacle  for 
those  few  fortunate  souls  who  were  lucky  enough  to  behold 
the  eclipsed  sun  under  good  conditions.  Elsewhere  along  the 
track  thin  haze  or  heavy  clouds  interfered  with  the  work  or 

1  Monthly  Notices  R.  A.  S.,  87,  657,  1927. 


ECLIPSES    SINCE    1923  219 

made  the  efforts  of  the  astronomers  of  no  avail.  At  Gallivare x 
in  Sweden,  there  were  several  expeditions,  the  party  from 
Upsala  securing  photographs  of  the  corona  and  of  the  flash 
spectrum.  These  observations  were  secured  through  thin 
clouds.  The  flash  spectrum  was  photographed  with  success 
by  Pannekoek  and  Minnaert,"  by  Vegard,  and  by  Baade. 

At  the  1925  eclipse  the  attempts  which  were  made  to 
photograph  the  phenomenon  in  color  met  with  partial  suc- 
cess. At  the  1927  eclipse  a  color  film  was  taken  at  Giggles- 
wick  making  a  brief  exposure  every  second  and  lengthening 
these  to  about  0.5  second  as  totality  approached.  The  results 
are  described  by  the  Astronomer  Royal.3  At  Giggleswick  the 
duration  of  totality  was  23  seconds.  For  27  of  the  exposures 
by  the  color  film  it  may  be  said  that  the  eclipse  was  total,  and 
for  an  additional  two  seconds  at  each  end  Baily  beads  are 
shown.  The  corona  and  prominences  can  be  traced  on  the 
limb  of  the  sun  opposite  to  the  crescent  for  a  duration  of  30 
seconds  both  before  and  after  totality.  The  corona  shows  on 
the  film  to  be  somewhat  bluish  in  color  during  totality. 
Outside  of  totality  this  bluish  color  appears  rather  reddish, 
the  difference  in  color  being  due  to  the  process  employed. 
The  chromosphere  appears  quite  red  in  color  while  the  promi- 
nences vary  from  a  deep  to  a  whitish  red. 

Before  totality  the  corona  appeared  in  some  places  bluish 
and  in  others  reddish.  When  totality  commenced  there  was 
a  continuous  reddish  arc  of  the  chromosphere  on  the  east 
limb,  extending  for  about  130°  with  red  prominences.  The 
chromosphere  was  visible  on  the  film  for  9  seconds  after  the 
beginning  of  totality  and  reappeared  again  6  seconds  before 
the  end  of  totality.  The  colors  on  the  film  do  not  agree  with 
those  seen  by  the  eye. 

On  account  of  the  short  duration  of  totality  the  bright 
inner  corona  was  left  exposed  beyond  the  dark  limb  of  the 
moon  for  a  longer  space  of  time  than  is  ordinarily  found  at 
a  total  eclipse  of  the  sun.  The  result  of  this  has  been  that 
the  corona  was  seen  and  photographed  for  a  much  longer 
stretch  of  time  outside  of  totality  than  is  usually  the  case. 

1  Kienle,  Die  Himmelswelt,  37,  225,  344,  1927. 

2  Verhand.  der  Kon.  Akad.  van  Wetenschappen  te  Amst'dm.,  13,  No.  5,  1928. 

3  Dyson,  Monthly  Notices  R.  A.S.,  88,  142,  1927. 


220  ECLIPSES    OF    THE    SUN 

References  to  other  features  of  the   1927  eclipse  will  be 
found  at  the  appropriate  places  in  the  following  pages. 

The  next  eclipse,  May  9,  1929,  was  visible  in  northern 
Sumatra,  the  Malay  States,  Siam  and  the  Philippines. 
Many  expeditions  were  attracted  to  the  eclipse  track  in  spite 
of  the  great  distances  from  home  that  must  be  traveled  and 
the  uncertainty  of  the  tropical  weather.  Professor  J.  A.  Mil- 
ler, a  veteran  of  many  eclipse  expeditions,  tells  1  of  the  gener- 
osity of  the  Dutch  in  caring  for  the  work  of  the  astronomers. 
All  of  the  heavy  equipment  was  carried  free  of  charge  by  the 
Dutch  steamship  lines  from  the  United  States  halfway  round 
the  world  to  Sumatra  and  return,  while  the  official  personnel 
of  the  expedition  was  given  passage  at  half  the  regular  rates. 
Unfortunately  the  Dutch  could  not  control  the  weather 
in  Takengon  in  northern  Sumatra  where  was  located  the 
Swarthmore  party  and  also  the  Potsdam  expedition  under 
Freundlich.  Miller  describes  the  weather  as  follows:  "We 
had  not  seen  a  clear  sun  for  days  preceding  the  eclipse.  It 
was  at  that  season  of  the  year  known  as  the  change  of  the 
Northeast  to  the  Southwest  Monsoon.  The  Southwest  Mon- 
soon was  to  bring  clear  weather,  and  it  did,  five  days  after 
the  eclipse. 

"The  eclipse  occurred  at  12:47  o'clock.  In  the  morning 
the  sky  was  entirely  overcast.  About  nine  o'clock  a  round 
patch  about  20°  in  diameter  appeared  in  the  southeast  sky. 
It  persisted,  and  seemed  to  drift  slowly  toward  the  com- 
pletely overcast  sun.  At  eleven  o'clock  one  could  tell  where 
the  sun  was  in  the  sky.  At  the  time  of  first  contact  the  sun 
was  invisible,  but  the  blue  circular  patch  still  drifted  slowly 
on.  Sometimes  during  the  partial  phase  one  could  tell  by 
using  field  glasses  that  the  sun  was  eclipsed.  Ten  minutes 
before  second  contact  we  took  our  places  at  the  instruments, 
having  decided  to  make  all  exposures  regardless  of  the 
clouds.  At  that  time  one  could  not  tell  with  the  unaided  eye 
that  the  sun  was  in  partial  eclipse.  Five  minutes  before 
totality  it  was  considerably  brighter,  and  at  totality  the  sun 
was  in  the  center  of  the  circular  blue  patch,  which  to  the 
naked  eye  seemed  perfectly  clear.  It  remained  clear  during 

1  Popular  Astronomy,  37,  495,  1929. 


ECLIPSES    SINCE    1923  221 

the  entire  period  of  totality,  then  the  clear  patch  moved  on, 
and  half  an  hour  after  totality  the  sun  was  again  obscured  by 
clouds." 

The  Swarthmore  party  secured  successful  photographs 
with  the  63-foot  tower  camera,  with  a  pair  of  1 5-foot  Ein- 
stein cameras  and  with  smaller  instruments.  The  German 
party  secured  excellent  photographs  for  the  Einstein  effect 
and  also  spectra  of  the  corona  in  fine  definition.  To  obtain 
check  plates  the  party  stayed  behind  at  the  eclipse  site. 
Observers  elsewhere  had  varied  success. 

Two  British  expeditions  were  located  at  Alor  Star  in 
Kedah  and  at  Pattani  in  Siam.  The  former  photographed 
through  haze,  the  latter  was  blotted  out  by  clouds.  Stetson, 
Arnold  and  Johnson  at  Alor  Star  with  an  illuminometer 
found  at  mid-totality  a  radiation  of  0.15  foot  candles.  The 
U.  S.  Naval  Observatory  expedition  was  at  Iloilo  in  the 
Philippines.  Good  photographs  of  the  corona  were  secured 
through  light  clouds.  In  Cebu  the  Hamburg  party  had  thin 
clouds;  Dutch,  French  and  Japanese  expeditions  were  par- 
tially successful. 

The  annular-total-annular  eclipse  of  April  28,  1930,  pre- 
sented special  problems.  In  consequence  of  the  usual  devia- 
tion of  the  moon  from  its  predicted  place,  would  it  be 
possible  for  an  expedition  to  succeed  in  locating  itself  inside 
the  path  of  totality  scarcely  more  than  a  half  mile  in  width? 
Evidently,  at  an  eclipse  of  this  character  the  most  fruitful 
field  of  investigation  would  be  the  flash  spectrum. 

Heavy  rain  and  clouds  along  the  track  in  California  and 
Nevada  greatly  interfered  with  the  observations.  With  their 
customary  good  fortune,  the  Lick  observers  had  a  clear  sky 
at  the  time  of  totality.  Two  prism  spectrographs  were  em- 
ployed each  with  moving  plate  according  to  the  method 
developed  by  Campbell.  The  photographs  by  both  instru- 
ments ]  recorded  the  lines  of  intermediate  and  high  levels  but 
the  timing  was  too  late  to  obtain  the  reversal  of  the  lines  of 
lowest  level.  The  expedition  from  Mount  Wilson  was  ham- 
pered bv  clouds.  Photographs  of  the  flash  spectrum  were 
obtained  by  two  concave  grating  spectrographs,  one  of 

i  Publ.  A.  S.  P.,  42*  131,  1930. 


222  ECLIPSES    OF    THE    SUN 

io-foot  radius  used  with  moving  plate  and  the  other  of 
2 1 -foot  radius  with  minified  images  of  the  crescents.  The 
latter  arrangement  designed  by  Anderson  was  used  at  the 
1926  and  1927  eclipses.  The  detailed  results  of  the  spectra 
from  this  brief  eclipse  will  be  watched  with  interest. 

The  path  of  the  recent  total  eclipse  started  at  sunrise  on 
October  22,  1930,  in  the  North  Pacific  and  ended  at  sunset 
off  the  coast  of  Patagonia  on  October  2 1 .  The  observation 
of  the  eclipse  was  dramatic  and  spectacular  in  all  of  its 
details.  The  only  available  site  was  a  small,  isolated,  vol- 
canic island  not  far  from  the  International  Date  Line  and 
at  15°  south  latitude.  The  U.S.  Naval  Observatory  secured 
a  small  appropriation  from  Congress  for  an  expedition.  As 
a  result  of  their  commendable  policy,  frequently  put  into 
effect  in  the  past,  astronomers  with  eclipse  experience  were 
invited  to  become  guests  of  the  expedition.  The  equipment 
consisted  of  115  cases  of  scientific  instruments  and  supplies, 
60  tons  of  stores  and  11,000  feet  of  lumber.  A  naval  vessel, 
U.S.S.  Tanager,  was  detailed  to  assist  in  the  work.  The 
members  of  the  party  were  on  the  island  two  months  before 
the  eclipse. 

The  island  of  Niuafoou  in  the  Tonga  group  is  familiarly 
known  as  "  Tin-Can  Island. "  The  island  being  both  small 
and  volcanic  there  are  no  bays  nor  landing  places  where  a 
boat  could  run  alongside.  Once  each  month  there  is  great 
excitement  on  the  island;  the  mail  steamer  arrives!  Unfor- 
tunately, the  only  sure  method  whereby  the  mail  can  be 
brought  ashore  is  to  seal  it  up  in  a  tin  can  and  lower  it  over 
the  ship's  side;  then  one  white  trader  and  two  natives  swim 
out  for  the  tin  can  —  at  the  same  time  carrying  the  out- 
going mail.  But  the  island  is  in  the  path  of  the  trade 
winds,  and  if  the  water  is  too  rough  to  permit  the  swim- 
mers to  go  out,  the  mail  steamer  does  not  stop,  and  the  island 
gets  along  without  news  from  the  outside  world  until  an- 
other month  rolls  by. 

The  island  is  shaped  like  a  gigantic  signet  ring.  It  has  a 
diameter  of  five  miles,  with  an  interior  lake  three  miles  in 
diameter  whose  surface  is  about  seventy-five  feet  above  sea 
level.  The  inhabitants,  uoo  in  number,  are  natives  of  the 


ECLIPSES    SINCE    1923  223 

Polynesian  race.  When  we  were  on  Niuafoou  the  white  male 
population  consisted  of  three  persons,  two  white  traders  and 
a  Catholic  priest. 

The  island  boasts  of  the  largest  cocoanuts  in  the  Pacific 
Ocean.  Copra  or  the  dried  ripe  cocoanut  is  the  only  article 
of  commerce.  When  a  boy  reaches  the  age  of  eighteen  he 
is  given  eight  acres  of  land  by  the  Government;  when  he 
marries  he  is  given  a  plot  of  ground  in  the  village  for  his 
home.  The  meat  of  the  young  green  cocoanuts  provides 
food  for  man,  dog,  chicken  and  pig,  and  the  water  is  used 
for  drinking  and  cooking,  and  also  for  the  preparation  of  the 
ceremonial  drink  of  kava. 

As  the  cocoanuts  grow  readily  without  any  attention,  life 
for  the  South  Sea  Islander  is  very  simple.  The  government 
taxes  must  be  paid  before  the  end  of  September  and  hence 
in  the  early  days  of  the  month  there  is  great  activity  in 
gathering  the  ripe  cocoanuts,  drying  the  meat,  and  then  sell- 
ing the  supply  to  one  of  the  two  trading  companies.  Then 
comes  the  tramp  steamer  from  the  outside  world  to  load  the 
copra  to  take  it  to  the  United  States  for  making  food  sup- 
plies and  soap.  When  the  taxes  are  once  paid  and  a  receipt 
obtained,  the  native  may  take  it  easy  for  the  balance  of  the 
year.  In  contrast  to  life  under  the  benefits  of  white  civiliza- 
tion, the  natives  lead  a  care-free  existence.  No  wonder  that 
Stevenson  and  others  have  written  such  fascinating  tales  of 
life  in  the  South  Seas!  We  enjoyed  greatly  the  friendly 
intercourse  on  our  brief  sojourn  on  Niuafoou.  At  four 
different  times  we  were  the  honored  guests  at  a  grand  ban- 
quet of  native  delicacies  followed  by  a  dance  when  we  were 
given  the  privilege  of  dancing,  American  fashion,  with  the 
native  belles.  The  daughter  of  the  head  man  of  the  village 
where  I  had  been  adopted  as  "  friend,"  rejoiced  in  the  eupho- 
nious name  of  Vaicima  —  which  translated  into  English 
means  "  cement  water-tank." 

The  erection  of  the  big  cameras  was  no  easy  task.  There 
was  the  63-foot  tower  telescope,  the  camera  of  65-foot  focus 
placed  horizontally  with  coelostat,  a  pair  of  Einstein  cameras 
of  1 5 -foot  focus  on  a  separate  equatorial  mounting,  together 
with  smaller  cameras.  All  of  this  was  under  the  direct 


224  ECLIPSES    OF    THE    SUN 

supervision  of  Professor  Ross  W.  Marriott  of  Swarthmore. 
I  had  two  concave  grating  spectrographs  used  at  former 
eclipses.  Fortunately,  Commander  Keppler  and  Dr.  Kellers 
of  the  U.  S.  Navy  had  been  at  the  eclipses  of  1926  and  1929, 
and  eclipse  problems  were  to  them  no  novelty.  Two  petty 
officers  and  eleven  enlisted  men  from  the  Navy  helped  in  the 
erection  of  the  apparatus.  A  month  before  the  eclipse  the 
Americans  were  joined  by  a  party  of  seven  from  New  Zealand 
under  the  direction  of  my  old  friend,  Dr.  C.  E.  Adams. 

Recent  eclipse  expeditions  to  the  tropics,  in  1926  and 
1929,  had  been  treated  rather  badly  by  the  weather.  What 
was  in  store  for  us?  Weather  records  kept  by  us  during  the 
eight  weeks  preceding  the  eclipse  had  shown  that  only  one 
day  out  of  three  was  clear  at  the  eclipse  hour.  On  eclipse 
day,  October  21  by  the  American  date  and  one  day  later 
according  to  local  reckoning,  conditions  before  the  great 
event  were  even  worse  than  usual.  Totality  was  to  occur  at 
8:51  in  the  morning.  Two  hours  before  this  a  light  rain  was 
falling;  first  contact  was  observed  through  clouds.  The 
clouds  soon  after  began  to  get  thinner  and  fifteen  minutes 
before  totality  they  had  entirely  cleared  away.  The  total 
phase  was  observed  under  nearly  perfect  conditions.  Pos- 
sibly there  was  a  slight  haze  but  this  in  no  way  seemed  to 
affect  the  photographs.  One  half  hour  after  totality,  clouds 
again  began  to  gather!  On  the  night  following  the  eclipse, 
Marriott  started  on  the  development  of  the  coronal  photo- 
graphs. On  the  following  night  the  dark  room  was  given 
over  to  my  exclusive  use.  As  soon  as  it  had  cooled  somewhat 
I  started,  and  with  the  help  of  ice  brought  for  the  purpose, 
I  was  able  to  keep  developing,  fixing  and  washing  baths  at  a 
fairly  satisfactory  temperature.  The  washing  of  the  plates 
was  difficult  on  account  of  the  scarcity  of  water.  On  the 
island  the  only  available  fresh  water  came  from  rains.  Un- 
fortunately there  had  been  practically  no  rain  during  our 
residence  on  Niuafoou  and  all  of  the  cisterns  in  the  vicinity 
of  the  eclipse  camp  were  dry.  Fresh  water  had  therefore 
to  be  brought  by  the  Tanager  in  eight-gallon  breakers  and  it 
was  necessary  to  conserve  this  in  every  possible  way.  That 
used  for  washing  the  photographic  plates  was  saved  and  was 


x 

si  « 

g  "H 


a 


o 

Pi 
O 
U 


w 
H 


ECLIPSES    SINCE    1923  225 

utilized  again  for  personal  use  and  for  dish  washing.  Lava 
dust  permeated  everywhere  and  it  was  difficult  to  dry  the 
plates,  especially  when  large  in  size,  without  their  being 
spoiled  by  dust  particles. 

When  all  of  the  photographic  plates,  direct  corona  and 
spectrographic,  were  developed,  the  eclipse  camp  was  a 
cheery  place.  We  had  been  fighting  against  tremendous  odds 
—  but  we  had  won  out.  I  have  never  seen  more  exquisite 
detail  than  is  found  on  the  coronal  photographs  taken  under 
the  direction  of  Marriott.  My  own  spectra  left  little  to  be 
desired,  with  good  focus  from  wave-length  3200  in  the  violet 
to  7800  in  the  red  and  with  the  region  from  4650  to  6800  in 
duplicate.  Measures  of  coronal  radiation  and  observations 
of  shadow  bands  will  be  discussed  in  later  pages. 

I  had  gone  to  Tin-Can  Island,  and  then  around  the  world 
on  the  return  journey,  to  work  for  ninety-three  seconds  of 
time.  On  August  31,  1932,  I  shall  observe  my  ninth  total 
solar  eclipse  at  a  location  in  Magog,  Quebec  —  compara- 
tively speaking,  almost  at  home.  There  also  will  be  my 
friend  Stratton  at  the  head  of  an  expedition  from  Cambridge, 
England,  and  Minnaert  from  Utrecht,  Holland.  Farther 
north  along  the  track,  at  Parent  in  northern  Quebec,  will  be 
an  expedition  from  the  Greenwich  observatory.  In  northern 
New  England  there  will  be  expeditions  from  the  two  Great 
California  observatories,  Mount  Wilson  and  Lick,  and  also 
parties  from  Swarthmore,  Van  Vleck  and  other  American  in- 
stitutions. The  eclipse  will  be  total  for  about  one  hundred 
seconds  of  time  and  the  path  will  be  approximately  100  miles 
in  width.  The  chances  of  clear  weather  appear  to  be  about 
fifty-fifty. 

By  referring  to  page  55  it  will  be  seen  that  if  I  am  ever 
to  observe  a  tenth  total  eclipse  of  the  sun  it  will  be  necessary 
for  me  to  travel  many  thousands  of  miles  from  my  home  in 
Virginia. 


CHAPTER  XIV 

THE  STRUCTURE  OF  THE  ATOM 

DURING  the  past  two  and  a  half  centuries,  the  as- 
tronomer and  the  mathematician  have  worked  in 
close  cooperation  investigating  the  distances  and 
motions  of  the  bodies  forming  the  sidereal  universe.  Under 
the  magic  wand  of  their  combined  labors,  the  complexity  of 
the  Ptolemaic  system  has  given  way  to  great  simplicity  and 
beautiful  order,  revealing  motions  obeying  the  inverse  square 
law  of  Newton,  or  the  very  slight  modification  of  this  law 
of  gravitation  demanded  by  the  theory  of  relativity.  Dur- 
ing the  progress  of  these  investigations,  our  conceptions  of 
distances  and  dimensions  have  been  gradually  modified,  so 
that  the  sidereal  universe  appears  to  be  vastly  greater  than 
was  formerly  thought.  Our  own  Galaxy  probably  has  a  radius 
exceeding  icr1  meters  or  one  hundred  thousand  light  years, 
while  beyond  extend  other  universes  at  distances  of  hundreds 
of  millions  of  light  years. 

While  the  astronomer  has  thus  been  reaching  out  to 
greater  and  yet  still  greater  distances  in  the  direction  to- 
ward the  infinite,  the  physicist  and  the  chemist,  on  the  other 
hand,  have  found  solar  and  planetary  systems  of  nearly 
infinitesimal  dimensions  within  the  realm  of  the  chemical 
atom.  The  radius  of  the  electron  we  seem  to  think  we  know 
is  equal  to  1.9  x  io-ir>  meters.  The  astronomer  has  con- 
tributed much  information  concerning  the  atom  because  the 
celestial  laboratories  of  sun  and  distant  stars  provide  high 
temperatures  and  minute  pressures  transcending  any  avail- 
able in  the  best-equipped  terrestrial  laboratories. 

The  amazing  development  of  our  conception  of  the  atom 
has  come  within  the  past  three  and  a  half  decades,  the  be- 
ginning taking  place  with  the  study  of  streams  of  negative 
corpuscles  or  electrons.  On  its  discovery,  radium  seemed 

226 


THE  STRUCTURE  OF  THE  ATOM      227 

superficially  to  exhibit  a  contradiction  of  the  laws  of  con- 
servation of  energy.  Heat  and  light  were  spontaneously 
emitted  without  any  apparent  changes  in  the  radium,  and 
thus  a  continuous  supply  of  energy  seemed  evolved  which 
set  at  naught  that  fundamental  law  of  physics.  But  it  was 
soon  found  that  with  the  giving  off  of  energy  in  radiation, 
the  radium  itself  did  utterly  change  and  here  the  philoso- 
pher's stone  seemed  at  last  to  have  been  discovered,  for 
it  was  found  that  one  chemical  element  actually  changed 
into  another. 

And  to  think  that  after  all  the  whole  science  of  radio- 
activity was  more  or  less  the  result  of  a  happy  accident! 
The  year  following  the  discovery  of  X-rays  in  1895  by 
Rontgen,  Becquerel  of  Paris  wished  to  test  the  phosphores- 
cent action  of  certain  substances  by  wrapping  a  photo- 
graphic plate  in  black  paper,  and  placing  on  it  the  substance 
to  be  examined,  which  was  then  exposed  to  sunlight.  By 
good  fortune  a  preparation  of  uranium  was  chosen  and  the 
photographic  plate  was  darkened.  The  Becquerel  rays  were 
thus  discovered,  and  it  was  soon  found  that,  like  the  X-rays, 
these  rays  penetrate  substances  impervious  to  light,  even 
passing  through  thin  plates  of  metal.  The  experiments  were 
always  made  by  placing  the  phosphorescent  substance  in  the 
sunlight  on  top  of  the  black  paper  enclosing  the  photo- 
graphic plate.  But  one  day  the  sun  was  clouded,  and  the 
plate  and  the  phosphorescent  substance  were  placed  away 
in  a  desk  and  were  left  there  for  several  weeks.  Becquerel 
for  some  reason  developed  the  plate,  and  was  surprised  to 
find  the  plate  darkened  as  before,  thereby  showing  that 
probably  neither  sunlight  nor  phosphorescence  had  any- 
thing to  do  with  the  action  on  the  photographic  plate.  Thus 
was  born,  in  1896,  the  new  science  of  radioactivity! 

Besides  the  effect  on  the  photographic  plate,  radioactive 
substances  manifest  themselves  in  three  different  manners: 
first,  by  exciting  phosphorescence  and  fluorescence;  second, 
by  causing  the  air  near  them  to  become  conductors  of  elec- 
tricity; but  most  startling  of  all,  by  the  continuous  genera- 
tion of  light  and  heat. 

Mme.  Curie  recognized  that  radioactivity  was  a  property 


228  ECLIPSES    OF    THE    SUN 

of  the  atom  and  starting  with  this  in  view  she  found  that 
the  residues  from  the  mine  at  Joachimstal,  Austria,  were 
three  to  five  times  more  radioactive  than  uranium.  From 
this  residue  she  separated  out  a  new  substance,  far  more 
active  than  uranium  which  she  called  polonium,  in  honor  of 
the  place  of  her  birth.  Later  she  discovered  radium.  This 
appears  to  be  an  element  with  atomic  weight  226,  and  it  is 
found  in  excessively  minute  quantities,  there  being  only  one 
part  in  five  million  in  the  best  pitchblende.  In  1899, 
Rutherford  showed  that  the  radiation  from  uranium  was 
complex,  consisting  of  (i)  the  a  rays,  which  are  absorbed 
by  a  sheet  of  paper  or  a  few  centimeters  of  air  (2)  of  a 
hundred-fold  more  penetrating  (5  rays,  capable  of  passing 
through  several  millimeters  of  aluminium,  and  (3)  of  still 
more  penetrating  y  rays,  capable  of  passing  through  quite 
a  thickness  of  iron  and  lead.  The  $  rays  are  deflected  by 
a  magnetic  field.  Becquerel  and  Kaufmann  showed  that 
the  (3  rays  were  negatively  charged  particles  projected  with 
a  velocity  approaching  that  of  light.  The  very  penetrating 
y  rays  are  not  deflected  in  a  magnetic  or  electric  field,  and 
are  probably  closely  connected  with  X-rays. 

Though  the  a  rays  are  the  least  penetrating,  they  are 
much  the  most  important  of  the  three  types  of  radiation. 
They  are  deflected  much  less  by  a  powerful  magnetic  field 
than  the  $  rays,  and  in  the  opposite  direction,  showing  that 
the  a  rays  consist  of  a  stream  of  positively  charged  particles. 
Alpha  rays,  therefore,  will  affect  a  gold  leaf  electroscope, 
and  this  old  instrument  gives  one  of  the  most  sensitive 
methods  of  measuring  the  amount  of  radiation.  In  fact, 
Rutherford  has  shown  that  it  is  not  difficult  to  measure  with 
certainty  the  presence  of  radium  in  a  body  which  contains 
as  small  a  quantity  as  io-n  grams  of  radium! 

The  maximum  velocity  of  the  a  rays  is  12,000  miles  per 
second.  These  a  rays  thus  move  with  velocities  hundreds  of 
times  greater  than  the  fastest  moving  meteor.  Everyone 
is  aware  of  the  enormous  energy  possessed  by  a  meteor 
moving,  say,  at  30  miles  per  second.  But  energy  varies  as 
the  square  of  the  velocity,  and  thus  the  a  particle  of  radium 
possesses  a  quarter  of  a  million  times  more  energy,  mass 


THE  STRUCTURE  OF  THE  ATOM      229 

for  mass,  than  a  swiftly  moving  meteor.  In  this  enormous 
energy  of  the  rays  lies  the  secret  6f  the  surprises  of  radium. 
From  whence  comes  this  enormous  store  of  energy? 

In  addition  to  its  power  of  sending  out  radiations,  radium 
possesses  another  important  property,  shared  in  by  the  radio- 
active substances  actinium  and  thorium,  namely,  that  of  con- 
tinuously emitting  a  radioactive  "  emanation  "  or  gas.  This 
property  is  rendered  very  striking  if  a  specimen  of  radium 
bromide  is  dissolved  in  water  and  the  liquid  evaporated 
down  to  dryness  again  to  get  the  solid  substance.  This 
simple  process  has  caused  the  radium  to  lose  the  greater 
part  of  its  radiation.  Strangely  enough  the  radium  slowly 
regains  its  activity,  and  if  left  entirely  to  itself,  at  the  end 
of  a  month  it  is  as  radioactive  as  ever.  Rutherford  has 
showed  that  the  solution  in  water  causes  the  radium  to  give 
off  a  gas  called  "  radium  emanation."  This  emanation  has 
all  the  properties  of  a  true  gas,  it  can  be  liquefied  at  a  tem- 
perature of  — 150°  C,  but  it  is  100,000  times  more  radio- 
active weight  for  weight  than  radium.  It  does  not  combine 
with  any  known  substance,  and  is  not  acted  upon  by  any 
chemical  reagent.  It  is  not  a  radium  compound,  but  it  is  a 
new  element  with  an  atomic  weight  which  appears  to  be  222. 
It  takes  its  place  along  with  the  rare  gases  of  the  air,  argon, 
helium,  neon,  etc.,  and  it  gives  a  characteristic  bright-line 
spectrum  which  shows  neither  the  radium  nor  helium  lines. 
It  seems,  therefore,  that  the  element  radium  has  been  trans- 
formed into  another  element,  radium  emanation,  or  radon. 
If,  after  a  month,  the  radium  is  again  dissolved  in  water  and 
evaporated  to  dryness  as  before,  the  radium  loses  its  ac- 
tivity, and  a  fresh  crop  of  emanation  is  produced.  This 
same  process  may  be  repeated  as  often  as  possible  with  the 
result  always  the  same,  and  we  are  perforce  compelled  to 
assume  that  the  radium  is  continually  manufacturing  emana- 
tion, continually  changing  itself  into  a  new  element.  This 
is  really  only  the  first  of  a  series  of  changes,  for  radium 
emanation  changes  into  radium  A,  and  this  in  turn  to  radium 
B,  and  so  on.  This  change  is  an  atomic  change  going  on 
within  the  atom.  But  how  does  this  change  progress? 

When  the  radium  has  given  off  the  emanation,  it  still 


230  ECLIPSES    OF    THE    SUN 

gives  out  a  particles,  but  only  about  one-fourth  as  copiously 
as  before  the  radium  was  put  in  water.  The  a  particles  are 
produced  by  the  same  change  as  makes  the  emanation,  and 
the  radium  atom  is  therefore  divided  into  emanation  and 
a  particle. 

Observations  of  the  velocity  and  mass  of  the  a  particle 
made  by  Rutherford  indicate  either  that  the  mass  of  the  a 
particle  is  twice  that  of  the  hydrogen  atom,  or  if  the  charge 
carried  by  the  a  particle  is  twice  that  of  the  hydrogen  atom, 
then  the  mass  of  the  a  particle  is  four  times  that  of  the 
hydrogen  atom  and  must  therefore  be  an  atom  of  helium. 
Hence  each  atom  of  radium  apparently  breaks  up  into  one 
atom  of  helium  and  one  of  radon. 

All  that  was  necessary  to  complete  Rutherford's  proof 
that  helium  was  actually  given  off  from  radium,  was  to  show 
experimentally  that  helium  was  thus  produced.  This  was 
accomplished  in  1903  by  Sir  William  Ramsay  and  Frederick 
Soddy.  A  tube  was  filled  with  radium  emanation  which  was 
separated  from  all  other  gases  by  condensing  it  with  liquid 
air  and  removing  by  a  pump  the  gas  not  condensed.  This 
spectrum  tube  was  sealed  and  the  spectrum  of  the  gas  could 
be  examined  at  will.  At  first  no  helium  lines  were  visible, 
but  after  a  lapse  of  three  or  four  days,  when  the  radium 
emanation  had  disintegrated,  the  spectrum  of  helium  gradu- 
ally made  its  appearance,  and  finally  the  whole  helium 
spectrum  was  complete.  Similarly,  Debierne  has  found  by 
the  spectroscope  that  helium  is  produced  from  the  radio- 
active substance  actinium,  and  Soddy  has  produced  helium 
from  uranium  and  thorium.  Helium,  therefore,  has  been 
found  experimentally  to  be  produced  by  the  radioactive  sub- 
stances radium,  thorium,  uranium  and  actinium.  These  sub- 
stances are  alike  in  that  each  emits  a  particles.  Hence, 
a  particles  are  atoms  of  helium.  Rutherford  and  Royds, 
however,  have  given  a  still  more  conclusive  proof  that  the 
a  particle  is  an  atom  of  helium.  These  a  particles  are  ca- 
pable of  penetrating  a  certain  small  but  definite  thickness  of 
glass.  Glass  may  be  blown  very  thin  but  yet  retain  its 
ability  to  remain  air  tight.  Radium  emanation  was  stored 
in  such  a  thin-walled  vessel  and  this  enclosed  in  a  second 


THE  STRUCTURE  OF  THE  ATOM      231 

vessel.  Alpha  particles  given  off  from  the  radium  emana- 
tion thus  could  penetrate  through  the  very  thin  glass  walls, 
but  were  stopped  in  the  outer  vessel  and  were  there  col- 
lected. At  first  the  gas  in  the  outer  vessel  was  found  to 
contain  no  helium,  but  after  some  days,  helium  lines  ap- 
peared in  the  spectrum,  proving  beyond  a  question  of  doubt 
that  radium  gives  off  helium. 

It  is  even  possible  to  measure  the  rate  of  growth  of  the 
helium,  which  measures  show  that  in  a  year,  168  cubic  milli- 
meters of  helium  are  spontaneously  manufactured  by  each 
gram  of  radium.  Rutherford  and  Geiger  in  this  connection 
achieved  one  of  the  greatest  triumphs  for  experimental 
science  in  being  able  to  count  the  number  of  helium  atoms 
or  a  particles  that  are  ejected  per  second  from  one  gram 
of  radium.  Indeed  two  different  methods  were  devised 
which  led  to  the  same  results.  Both  methods  depend  on  the 
fact  that  each  atom  of  helium  as  it  is  ejected  gives  a  small 
flash  like  a  meteor.  By  an  electrical  method,  these  flashes 
were  counted  by  Rutherford  and  Geiger  and  it  was  found 
by  them  that  thirty-four  thousand  million  (3.4  x  io10)  atoms 
of  helium  are  ejected  every  second  from  each  gram  of 
radium.  This  number  is  in  exact  agreement  with  that  ob- 
tained by  noting  with  a  microscope  the  number  of  scintilla- 
tions on  a  given  area  in  a  given  time  by  the  spinthariscope, 
invented  by  Sir  William  Crookes.  Thus  at  the  same  time 
there  was  measured  the  amount  of  helium  produced  from 
radium,  and  likewise  was  given  the  number  of  molecules 
present  in  matter,  information  which  was  needed  to  com- 
plete many  theories  in  physics. 

Investigations  in  radioactivity  accordingly  have  given  an 
entirely  new  conception  of  the  atom.  The  atom  is  no  longer 
one  and  indivisible,  but  certain  atoms  at  least  are  trans- 
formed into  other  atoms,  each  radium  atom  being  changed 
into  one  atom  of  helium  and  one  of  radium  emanation. 
These  atoms  are  continually  changing,  no  less  than  thirty- 
four  thousand  million  atoms  of  helium  being  produced  each 
second  of  time  from  each  gram  of  radium.  As  the  atoms 
disintegrate,  enormous  stores  of  energy  are  let  loose,  and 
this  energy  manifests  itself  as  light  and  heat.  The  heating 


232  ECLIPSES    OF    THE    SUN 

effect  of  this  energy  has  been  measured  and  has  been  found 
to  be  118  gram-calories  per  hour  per  gram  of  radium.  A 
specimen  of  a  grain  of  radium  bromide  would  evolve  about 
four  calories  per  hour.  In  four  years  about  140,000  calories 
would  have  been  evolved.  An  equal  weight  of  coal  would 
during  complete  combustion  give  out  about  500  calories. 
Hence  the  radium  in  four  years  would  give  280  times  as 
much  heat  as  if  it  had  been  coal  and  had  been  completely 
burned,  and  yet  the  radium  in  this  time  would  diminish  so 
very  little  in  weight  that  it  would  be  absolutely  impossible 
to  detect  this  diminution  by  the  most  sensitive  balance 
known  to  modern  science.  The  energy  of  radium  comes 
from  the  disintegration  of  its  atoms.  The  average  life  of  a 
radium  atom  is  2280  years,  so  that  in  the  complete  life  of 
one  grain  of  radium  about  100,000,000  calories  are  set  free. 
This  is  200,000  times  more  energy  than  if  it  were  pure  coal 
and  entirely  burned ! 

Helium,  being  permanent  and  not  transitory,  must  ac- 
cumulate as  the  result  of  radioactive  changes.  In  these 
changes,  Soddy  has  shown  the  remarkable  sensitiveness  of 
the  spectroscope  in  detecting  slight  quantities  of  helium 
for  he  has  proved,  in  numerous  special  experiments,  that  the 
D.<  line  of  the  helium  spectrum  can  be  detected  with  cer- 
tainty, if  only  one  millionth  of  a  cubic  centimeter,  or  one 
five-thousand-millionth  part  of  a  gram  of  helium  is  present. 

An  achievement  of  far-reaching  importance  in  all  theories 
of  physics  was  the  discovery  by  Sir  J.  J.  Thomson  of  a 
body  having  a  mass  much  less  than  that  of  the  lightest 
known  atom,  hydrogen.  This  body,  called  by  its  discoverer 
a  corpuscle,  but  now  known  as  an  electron,  is  i/i845th  part 
of  the  mass  of  the  hydrogen  atom.  A  further  study  of  the 
electron  showed  that  it  is  always  associated  with  a  negative 
charge  of  electricity  and  in  fact  carries  a  unit  charge  of 
negative  electricity.  The  physicist  has  now  become  con- 
vinced that  the  atom  is  an  electrical  structure  made  up  of 
nearly  equal  amounts  of  positive  and  negative  electrical 
charges.  The  atom  is  believed  to  consist  of  a  central  group 
of  elementary  positive  charges,  or  protons,  with  a  smaller 
number  of  negative  charges,  or  electrons,  called  the  nucleus, 


THE  STRUCTURE  OF  THE  ATOM      233 

and  about  this  nucleus  there  is  an  outer  system  of  negative 
electrons,  varying  in  number  from  one  to  ninety-two.  These 
outer  electrons  can  be  expelled  from  the  atom  by  a  number 
of  different  methods,  such  as  the  application  of  heat,  impact 
of  ions,  exposure  to  ultra-violet  or  X-rays,  or  they  may  be 
emitted  by  radioactive  substances;  in  fact,  the  (3  rays  con- 
sist of  a  stream  of  negatively  charged  particles. 

The  discovery  by  C.  T.  R.  Wilson  that  the  charged  ions 
produced  in  gases  by  a  and  p  rays  become  the  centers  for 
the  condensation  of  water  vapor  paved  the  way  to  experi- 
mental work  of  a  remarkable  nature.  Millikan  has  secured 
extraordinary  results  by  utilizing  tiny  drops  of  oil  in  place 
of  water,  and  as  a  result  of  his  experiments,  he  has  been 
able  to  prove  conclusively  that  the  electrical  charges  carried 
on  ions  "  all  have  the  same  value  or  else  small  exact  mul- 
tiples of  that  value. "  This  fundamental  unit  is  the  same, 
both  for  positive  and  negative  electricity,  and  is  numerically 
equal  to  the  charge  carried  by  the  negative  electron.  This 
unit  charge  of  electricity  was  measured  by  him  to  an  ac- 
curacy of  one  part  in  1000.  With  this  information,  it  was 
possible  to  estimate  with  greater  accuracy  the  mass  of  the 
electron  in  grams  and  the  number  of  molecules  of  any  gas 
per  cubic  centimeter  at  o°  C  and  760  mm  pressure. 

Since  we  know  therefore  the  size  of  a  molecule  and  the 
number  of  molecules  per  cubic  centimeter,  it  is  possible  to 
compute  the  number  of  molecules  through  which  the  a  or 
the  (J  rays  emitted  by  radium  must  pass  in  going  a  given  dis- 
tance. The  extraordinary  fact  revealed  (by  the  photographs 
of  Wilson,  referred  to  above)  is  that  the  swift-moving  (J  par- 
ticles pass,  on  the  average,  through  as  many  as  10,000  atoms 
before  coming  close  enough  to  an  electron  to  detach  it  from 
its  system  and  form  an  ion.  In  fact,  it  has  been  shown  by 
Eddington  L  that  when  an  electron  encounters  an  ionized 
atom  it  will  be  captured  if,  and  only  if,  it  actually  hits  the 
nucleus  of  the  atom.  The  electron  must  therefore  form 
but  a  very  minute  portion  of  the  space  enclosed  within  the 
atomic  system.  The  a  particle  being  an  atom  of  helium  with 
a  mass  more  than  seven  thousand  times  that  of  the  negative 

1  Monthly  Notices,  R.  A.  S.,  83,  32,  1922. 


234  ECLIPSES    OF    THE    SUN 

electron,  it  cannot  be  deflected  from  its  course  by  an  elec- 
;ron  which  is  of  very  minute  mass,  but  only  by  some  ponder- 
ible  mass  at  least  comparable  with  that  of  the  helium  atom. 
This  heavier  mass  is  found  at  the  nucleus  of  the  atom.    As 
i  result  of  Geiger  and  Marsden's  experiments  on  the  scat- 
ering  of  a  rays,  it  was  found  that  when  these  rays  passed 
hrough  very  thin  metallic  foils,  the  deflections  witnessed 
:ould  be  explained  only  by  assuming  a  very  close  approach  to 
i  small  but  massive  charged  particle.     Rutherford  x  was 
tccordingly  led  to  assume  that  the  typical  model  of  an  atom 
onsisted  of  an  exceedingly  minute  and  comparatively  mas- 
ive  positively  charged  nucleus,  about  which  is  collected  a 
lumber  of  electrons.    Each  and  every  one  of  the  electrons 
orming  the  outermost  parts  of  all  atoms  are  exactly  alike 
,nd  each  carries  a  unit  amount  of  negative  electricity.    Since 
ach  atom  is  electrically  neutral,  the  charge  on  the  positive 
lucleus  must  be  equivalent  to  the  sum  of  those  carried  on  the 
J  electrons.    The  value  of  N  for  each  of  the  atoms  is  a  f unda- 
lental  constant,  for  on  it  depends  the  size  of  the  electric 
ield  surrounding  the  nucleus  and  the  peculiar  arrangement 
f  the  external  electrons,  which  in  turn  determine  the  physi- 
al  and  chemical  properties  of  the  atom.     Experiments  in 
911,  by  Barkla  on  the  scattering  of  X-rays  indicated  that 
he  number,  N,  of  electrons  in  an  atom  was  approximately 
alf  the  atomic  weight  of  the  element.    This  conclusion  was 
bundantly  verified  by  the  magnificent  work  of  Moseley. 
le  found  that  the  X-ray  spectrum  was  similar  for  all  ele- 
lents,  and  that  when  he  plotted  the  square  root  of  the 
requencies  of  the  characteristic  X-ray  spectra,  all  the  ele- 
lents  examined  arranged  themselves  upon  nearly  perfectly 
:raight  lines.    The  atoms  were  then  numbered  in  the  order 
i  which  these  spectra  placed  them  to  give  these  straight 
nes. 

The  ordinal  number  corresponding  to  the  place  occupied 
y  each  element  in  the  periodic  table  (p.  306)  has  been 
>rmed  its  atomic  number.  The  work  of  Moseley  showed 
lat  all  the  chemical  elements  took  their  proper  places  in 
le  periodic  table,  and  as  uranium  is  the  heaviest  atom 

1  Phil.  Mag.,  21,  669,  1911,  and  27,  488,  1914. 


THE  STRUCTURE  OF  THE  ATOM      235 

known  (atomic  number  92)  there  can  be  only  92  species  of 
elements.  With  elements  85  and  87  now  thought  to  be 
known,  all  gaps  have  been  filled  in  the  periodic  table. 

These  discoveries  of  the  physicist  have  been  the  greatest 
boon  to  the  work  of  the  chemist.  The  latter  has  always  had 
great  faith  in  the  principles  underlying  the  Mendeleeff  table, 
but  in  this  table,  arranged  in  order  of  increasing  atomic 
weights,  certain  discrepancies  appeared;  for  instance,  argon, 
of  atomic  weight  39.88,  from  its  properties  was  compelled 
to  find  a  place  in  the  table  before  potassium  having  a  smaller 
atomic  weight  of  39.1.  The  system  of  atomic  numbers 
places  A  with  number  18,  in  its  rightful  place  before  K  with 
atomic  number  19.  In  a  similar  way,  the  system  of  atomic 
numbers  places  Co  in  the  table  before  Ni,  instead  of  after 
it,  if  arranged  according  to  atomic  weights. 

With  a  knowledge  of  atomic  numbers,  the  difficulties  of 
classification  presented  by  radioactive  substances  were  now 
cleared  away.  Soddy  has  shown  that  when  an  a  particle  is 
emitted,  the  position  of  the  element  in  the  periodic  table 
is  shifted  by  two  numbers  to  the  left  towards  smaller  num- 
bers, while  if  a  [4  particle  is  emitted  the  atomic  number  in- 
creases by  one  unit,  and  the  place  is  shifted  one  to  the  right. 
Since  the  a  particle  is  an  atom  of  helium  with  positive  charge 
2  and  mass  4,  while  the  (3  particle  is  a  negative  electron  with 
no  appreciable  mass,  it  is  evident  that  the  emission  of  an  a 
particle  will  diminish  the  atomic  weight  by  4,  but  the  emis- 
sion of  a  (J  particle  will  cause  no  change  in  the  atomic  weight. 
Consequently,  if  the  emission  of  an  a  particle  by  a  substance 
is  followed  by  two  successive  changes  in  which  (J  rays  are 
set  free,  the  net  result  will  be  that  the  element,  after  ex- 
periencing these  three  changes,  will  move  back  again  into 
the  position  in  the  periodic  table  it  had  held  originally. 
These  changes  have  not  altered  the  size  of  the  atomic  num- 
ber, but  have  diminished  the  atomic  weight  by  4,  and  con- 
sequently it  is  possible  for  two  or  more  elements  to  have 
the  same  atomic  number  but  differ  in  their  atomic  weights. 
Such  elements  are  known  as  isotopes.  Isotopes  are  indistin- 
guishable from  each  other  by  any  chemical  tests,  or  by  any 
spectroscopic  tests  since  the  spectra  are  identical.  Radium 


236  ECLIPSES    OF    THE    SUN 

(at.  wt  226),  Th  X  (at.  wt.  224)  and  Ac  X  (at.  wt.  222) 
are  examples  of  isotopes,  each  possessing  a  nuclear  charge  or 
an  atomic  number  of  88. 

Since  all  masses  are  nothing  more  than  electromagnetic 
manifestations,  and  since  the  mass  of  the  electron  is  very 
minute  and  negligible  compared  with  the  mass  of  the  nu- 
cleus, it  should  be  possible  to  compare  the  masses  of  dif- 
ferent elements  by  subjecting  them  to  successive  electric  and 
magnetic  fields.  This  method  of  "  positive  ray  analysis  " 
developed  by  J.  J.  Thomson  consists  in  measuring  the  ratio 
of  charge  to  mass.  This  was  made  possible  by  a  beautiful 
photographic  method,  which  in  the  capable  hands  of  Aston 
has  greatly  improved  our  knowledge  regarding  the  atoms.  Al- 
though demanding  technical  skill  of  a  very  high  order,  the 
chemical  examinations  can  be  carried  on  by  simple  methods 
leading  to  definite  results.  A  small  supply  only  of  the  gas 
to  be  investigated  is  required ;  it  need  not  be  chemically  pure 
nor  need  any  special  care  be  taken  to  wash  away  from  the 
vacuum  tube  all  traces  of  the  last  gas  investigated.  Ac- 
cording to  Aston,  it  is  impossible  to  remove  from  a  tube  "  all 
visible  traces  of  a  misspent  career."  Until  quite  recently 
it  was  thought  that  the  experimental  measurements  proved 
that  the  elements  whose  atomic  numbers  are  whole  num- 
bers, with  oxygen  assumed  at  16.00,  are  pure  elements,  while 
all  other  elements  with  fractional  atomic  weights  are  mix- 
tures of  isotopes,  each  of  the  isotopes  however  having  a 
whole  number  for  its  atomic  weight.  B  (at.  wt.  10.9)  is  a 
mixture  of  two  isotopes  of  masses  n  and  10;  Ne  (at.  wt. 
20.2)  consists  of  two  isotopes,  masses  20  and  22;  Mg  (at. 
wt.  24.32)  is  a  mixture  of  three,  of  masses  24,  25  and  26; 
Cl  (35.46)  consists  of  two,  of  masses  35  and  37;  while  Xe 
and  Hg  are  each  made  up  of  no  less  than  6  isotopes.  This 
conclusion  has  been  greatly  modified  by  the  discovery  that 
each  of  the  elements  C,  H  and  O  has  isotopes. 

Our  view  is  that  the  nuclei  of  all  atoms  are  made  up  of 
multiples  of  hydrogen  nuclei  each  carrying  unit  positive 
charge,  the  combination  being  bound  together  by  the  exter- 
nal electrons.  Hence  the  mass  of  the  atom  is  confined  to  the 
nucleus,  but  the  size  of  the  nucleus  is  very  minute  compared 


THE  STRUCTURE  OF  THE  ATOM      237 

with  the  whole  volume  of  the  atom.  In  fact,  the  radius 
of  an  electron  cannot  be  larger  in  comparison  with  the  radius 
of  the  atom  than  is  the  radius  of  the  earth  compared  with 
the  distance  from  earth  to  sun.  Each  atom  therefore  forms 
a  miniature  solar  system,  the  external  electrons  being  held 
in  place  and  compelled  to  perform  their  orbital  motions  by 
the  comparatively  massive  nucleus.  Since  there  may  be  as 
many  as  92  external  electrons,  it  is  evident  that  modern 
mathematics  cannot  furnish  a  general  solution  of  the  motions 
of  the  electrons,  except  in  the  case  of  the  very  simplest  of  the 
atoms. 

Since  the  chemical  and  physical  properties  depend  on  the 
distribution  of  the  electrons  of  the  outer  atom,  there  have 
been  many  attempts  to  formulate  a  structure  for  the  atom. 
From  such  attempts  have  gradually  evolved  atomic  models 
of  many  different  types.  The  Lewis-Langmuir  atom  has 
been  very  successful  in  explaining  the  chemical  properties, 
particularly  the  valence.  This  atom  is  not  based  on  any 
dynamic  principles.  Valence,  which  measures  the  power  to 
combine,  may  be  positive  or  negative  depending  on  whether 
the  atomic  system  has  too  many  or  too  few  electrons  to 
make  a  stable  combination.  The  inert  gases  —  helium,  neon, 
argon,  krypton,  xenon  and  radon  —  are  elements  which  have 
no  power  to  form  compounds.  Such  atomic  systems  under 
ordinary  conditions  cannot  capture  an  electron  from  an- 
other atom,  nor  can  they  get  rid  of  one  of  their  own.  The 
inert  gases  have  atomic  numbers  of  2,  10,  18,  36,  54  and  86, 
and  hence  we  may  imagine  the  atoms  as  if  made  up  of  con- 
centric shells  of  electrons,  the  shells  containing  2,  8,  8,  18, 
1 8  and  32  electrons  respectively. 

Except  for  the  hydrogen  system,  all  atomic  structures 
have  the  two  electrons  forming  the  system  of  helium  as 
their  innermost  shell.  Since  helium  is  inert  and  cannot  take 
up  another  electron;  the  third  electron  which  forms  lithium 
must  be  a  single  electron  in  a  shell  exterior  to  the  helium 
system.  The  lithium  system  is  therefore  not  very  stable, 
and  readily  gives  up  its  external  electron.  It  has  a  positive 
valence  of  one.  The  system  of  fluorine,  with  seven  electrons 
in  the  second  shell,  may  be  regarded  as  being  in  a  position 


238  ECLIPSES    OF    THE    SUN 

readily  to  capture  an  electron  from  an  atom  in  its  neigh- 
borhood, and  so  it  has  a  positive  valence  of  seven  or  negative 
valence  of  one.  Likewise  oxygen  may  be  regarded  as  having 
positive  or  negative  valences  of  6  and  2  respectively.  The 
Lewis-Langmuir  atom  is  very  successful  in  explaining  the 
relative  positions  of  the  elements  in  the  periodic  table. 
The  electrons  forming  the  atom  of  this  model  are  relatively 
fixed  in  position,  for  though  each  electron  may  be  in  motion, 
it  is  confined  to  a  small  portion  of  the  space  occupied  by  the 
atom.  With  the  advance  in  knowledge  this  atomic  model 
has  been  superseded  by  others. 

Great  difficulties,  however,  appear  when  discussing  atomic 
models  on  the  principles  of  physics,  especially  when  there  is 
a  transference  of  energy  from  place  to  place,  or  when  the 
motions  of  the  atoms  make  them  depart  from  the  steady 
state  thereby  causing  emission  or  absorption  of  light.  After 
repeated  failures  to  explain  these  matters  by  means  of  the 
accepted  theories,  Planck  made  one  of  the  most  startling 
proposals  ever  presented  to  the  scientific  world,  from  which 
developed  his  celebrated  quantum  theory.  According  to 
Planck,  it  was  assumed  that  the  transference  of  energy  can 
only  take  place  in  definite  but  very  small  units,  and  that 
the  total  energy  transferred  is  always  an  integral  multiple 
of  this  small  unit,  called  the  energy  quantum.  This  can 
be  expressed  simply  in  mathematical  terms.  If  Ei  and  E2 
represent  the  energy  of  a  system  before  and  after  radiation 
has  taken  place,  then  the  energy  spent  in  radiation  is 
Ei  —  E>  =  hv,  where  h  is  Planck's  constant  and  v  is  the  fre- 
quency of  vibration  of  the  body  concerned.  This  mathe- 
matical equation  gives  expression  to  the  simple  statement 
that  the  total  amount  of  energy  emitted  or  absorbed  by  a 
radiating  body  is  always  proportional  to  the  frequency  of 
vibration,  which  in  turn  is  inversely  proportional  to  the 
wave-length  of  the  light  emitted  or  absorbed. 

The  very  radical  nature  of  Planck's  hypothesis  may  be 
estimated  when  it  is  stated  that  it  stands  at  variance  with 
all  the  previously  known  laws  of  mechanics  developed  to 
explain  the  motions  of  material  objects  of  large  dimensions. 
As  a  matter  of  fact,  there  is  no  absolute  necessity  that  the 


THE  STRUCTURE  OF  THE  ATOM      239 

same  laws  that  apply  to  ponderable  material  in  a  gross  state 
must  also  be  applicable  to  simple  atoms.  This  duality  of 
laws,  a  topic  of  much  heated  discussion  by  physicists  in  re- 
cent years,  has  given  rise  to  the  expression  "  classical  dy- 
namics "  which  explains  the  motions  of  matter  in  obedience 
to  the  law  of  gravitation.  One  fundamental  conclusion  of 
the  quantum  theory  is  that  motion  is  not  the  continuous 
process  that  we  have  accustomed  ourselves  all  our  lives  to 
believe;  but  the  motion  takes  place  "  steadily  by  jerks/' 
the  jerks  however  being  so  small  that  the  process  is  to  all 
practical  purposes  continuous. 

Of  the  many  atomic  models  proposed,  the  most  successful 
in  explaining  the  physical  facts,  and  more  particularly  the 
spectroscopic  data,  is  the  Bohr-Sommerfeld  atom.  This  was 
first  proposed  in  1913  and  it  is  based  on  Planck's  quantum 
theory  of  energy.  In  explaining  the  motions  of  the  external 
electrons,  evidently  little  hope  can  be  expected  from  the 
classical  dynamics,  for  this  would  require  the  solution  of  the 
problem  of  n  bodies  when  n  is  large.  For  the  present,  as- 
sumptions must  be  made  for  the  purpose  of  securing  results, 
and  in  spite  of  much  inconsistency,  the  quantum  theory  of 
spectra  is  the  most  satisfactory  attempt  so  far  made  to- 
ward interpreting  spectral  series.  In  the  simple  case  of  the 
hydrogen  atom  and  ionized  helium,  each  with  one  external 
electron,  the  Bohr-Sommerfeld  method  has  been  very  suc- 
cessful in  reproducing  many  of  the  details  of  the  spectra 
both  in  electric  and  magnetic  fields.  On  the  basis  of  this 
theory,  the  problem  of  atomic  structure  consists  in  building 
up  each  atom  in  such  a  way  that  the  passage  of  an  electron 
from  one  stationary  state  to  another  will  give  the  observed 
wave-lengths.  Although  the  acceptance  of  the  principles  of 
wave  mechanics  gives  a  better  explanation  of  the  theory  of 
spectra,  nevertheless  the  Bohr  atomic  model  is  still  useful 
in  explaining  the  facts. 

In  the  investigations  of  spectra,  the  first  of  the  lines  to 
show  an  arrangement  in  series  were  those  due  to  hydrogen, 
discovered  in  1880  by  Huggins  in  the  spectra  of  white  stars. 
This  series  is  an  extension  of  the  four  lines  visible  in  the 
solar  spectrum.  A  new  era  in  spectroscopy  started  in  1885 


240  ECLIPSES    OF    THE    SUN 

when  the  law  underlying  the  hydrogen  series  was  discovered 
by  Balmer.  The  thirty-five  lines  found  in  the  flash  spectrum 
are  represented  by  the  formula 

X  =3646.125-^- 


where  X  is  the  wave-length  on  Rowland's  scale  and  m  takes 
the  values  3,  4,  5.  ... 

Shortly  afterwards,  Kayser  and  Runge,  and  Rydberg  in- 
dependently, began  the  publication  of  their  splendid  re- 
searches. Rydberg's  investigations  are  of  the  greatest  im- 
portance since  they  have  laid  the  foundation  for  all  future 
work  on  spectra  in  series.  He  began  by  sorting  out  doub- 
lets and  triplets  and  thus  ascertaining  the  lines  which  belong 
together  in  a  series.  He  was  able  to  distinguish  three  chief 
kinds  of  series,  as  follows: 

Principal,  including  the  strongest  lines 
Diffuse,  of  intermediate  intensity 
Sharp,  including  the  weakest  lines. 

Each  of  these  three  series  may  consist  of  single,  double  or 
triple  lines.  Each  and  every  series  always  converges  to- 
wards a  limit  at  short  wave-lengths,  and  the  lines  at  the 
same  time  diminish  in  intensity.  A  fourth  so-called  "  fun- 
damental "  series  with  lines  mainly  in  the  infra-red  has  been 
discovered  by  Bergmann. 

Many  and  varied  are  the  mathematical  formulae  em- 
ployed to  represent  the  series  of  spectra.  The  most  satis- 
factory formula,  which  is  due  to  Rydberg,  takes  the  following 
form: 

A  N 

Vm     =    A     - 


(m  +  ju)2 

where  A  is  the  limit  of  the  series,  N  is  the  "  Rydberg  con- 
stant" for  hydrogen,  and  the  wave  numbers  vm  are  ob- 
tained by  assigning  successive  integral  values  to  m.  \i  may 
be  regarded  as  a  decimal  part  to  m,  though  it  is  sometimes 
greater  than  unity.  For  the  details  of  the  investigations  of 


THE  STRUCTURE  OP  THE  ATOM     241 

series  spectra,  one  would  do  well  to  read  Fowler's  excellent 
Report  on  Series  in  Line  Spectra,  1922. 

Of  special  interest  in  dealing  with  the  flash  spectrum  are 
the  investigations  regarding  enhanced  lines.  Fowler  has 
shown  l  that  these  lines  form  series  entirely  similar  to  those 
of  the  ordinary  lines.  The  formula  representing  the  en- 
hanced series,  however,  differs  from  that  of  the  ordinary 
series  in  that  the  Rydberg  constant  N  is  multiplied  by  4. 
This  has  a  simple  explanation  from  Bohr's  theory.  The  or- 
dinary series  lines  are  emitted  when  an  electron  of  charge  e 
returns  to  an  atom  from  which  it  has  been  displaced.  The 
enhanced  lines,  on  the  other  hand,  are  produced  when  an 
electron  returns  to  the  atom  which  has  already  lost  another 
electron  through  ionization;  consequently,  two  electrons 
are  detached  from  the  atom,  each  electron  carrying  a  charge 
e,  or  a  total  charge  2e.  The  formula  for  the  Rydberg  con- 
stant TV,  involves  the  square  of  the  charge  e,  and  hence  for 
the  enhanced  lines  the  multiple  4  appears.  Enhanced  lines 
therefore  belong  to  the  ionized  atom,  or  one  which  has  lost 
a  negative  electron  and  hence  carries  an  excess  of  positive 
charge.  According  to  a  suggestion  by  Saunders,  He-\-  and 
C0+,  refer  to  ionized  helium  and  calcium,  the  addition  of 
the  +  sign  following  the  chemical  symbol  signifying  that  the 
atom  is  not  electrically  neutral  but  carries  a  unit  +  charge. 
After  an  atom  has  lost  one  electron  and  thus  becomes  ion- 
ized, it  may  lose  a  second  electron  and  become  "  doubly 
ionized."  The  symbol  adopted  for  calcium  under  these  con- 
ditions is  Ca++?  the  atom  carrying  two  extra  positive 
charges.  According  to  Bohr's  theory,  the  charge  concerned 
in  the  production  of  spectrum  lines  by  such  an  atom  is  30, 
and  hence  the  Rydberg  constant  TV  must  be  multiplied  by  9. 

This  is  not  the  place  to  give  the  mathematical  theories 
underlying  the  formation  of  spectrum  lines,  but  a  synopsis 
may  be  given  of  the  more  important  developments  found  in 
Fowler's  Report  on  Series  in  Line  Spectra,  1922,  and  in 
Russell,  Dugan  and  Stewart's  Astronomy,  1927.  Any  ac- 
cepted theory  must  explain  why  the  frequency  of  any  line  in  a 
spectrum  appears  always  as  the  difference  between  the  terms 

1  Phil.  Trans.  A.  214,  225,  1914. 


242  ECLIPSES    OF    THE    SUN 

of  a  quantity,  neither  of  which  represents  a  spectral  line, 
and  must  furnish  an  explanation  of  the  physical  meaning 
of  the  two  terms,  and  must  further  explain  how  an  emitted 
frequency  comes  to  be  the  difference  of  two  of  these. 

Adopting  the  idea  of  Rutherford,  outlined  above,  that 
each  atom  consisted  of  a  heavy  nucleus  carrying  a  positive 
charge  which  was  surrounded  by  negative  electrons,  Bohr 
was  able  to  give  a  satisfactory  theory  in  the  case  of  the  sim- 
plest type  of  atom.  The  hydrogen  atom  is  such  a  unit  since  it 
consists  of  a  single  electron  in  orbital  motion  around  the  nu- 
cleus, and  equally  simple  are  the  atoms  of  enhanced  helium 
and  doubly  enhanced  lithium.  According  to  the  Bohr 
theory,  the  single  external  electron  is  free  to  traverse  cer- 
tain specified  orbits,  which  are  determined  in  the  simple 
case  of  circular  orbits  by  the  condition  that  the  angular 
momentum  is  an  integral  multiple  of  h/2  IT,  where  h  is 
Planck's  constant,  derived  from  the  quantum  theory.  When 
the  motion  of  the  electron  is  confined  to  one  of  these  sta- 
tionary orbits,  there  is  no  radiation.  Emission  occurs 
only  when  the  electron  passes  from  one  stationary  orbit  to 
another.  Without  attempting  to  explain  the  mechanism 
which  causes  the  electron  to  pass  from  orbit  to  orbit,  Bohr 
supposes  that  the  transition  is  followed  by  the  emission  of 
light,  the  frequency  of  which  can  be  determined  from  the 
quantum  theory.  In  fact,  the  energy  radiated  is  equal  to 
the  differences  of  the  energies  of  the  electron  in  the  two 
orbits  concerned,  and  is  assumed  to  be  one  quantum  of 
energy,  hv,  where  h  is  again  the  Planck  constant  and  v 
is  the  frequency.  At  a  given  instant,  any  one  electron  falling 
from  an  external  to  an  internal  orbit  causes  one  line  only 
in  the  spectrum,  and  it  is  the  summation  of  the  actions  of  a 
large  number  of  electrons  that  causes  the  whole  series  of 
spectrum  lines. 

By  the  elementary  laws  of  mechanics,  it  is  possible  to  de- 
rive the  necessary  equations  for  orbital  motion.  Taking 
E  and  M  as  the  charge  and  mass  of  the  nucleus,  e  and  m  as 
the  charge  and  mass  of  the  electron,  c  the  velocity  of  light, 
and  v  the  wave-number  of  the  line,  then  for  the  case  of 
hydrogen 


THE    STRUCTURE    OF    THE    ATOM 


243 


2?rz  Jf  e      (i        i 

v  = —  m(-~-  -  — 

ch*          W      /22 

where  ti  and  /2  are  integers. 

This  formula  is  of  exactly  the  same  form  as  that  which 
represents  the  Balmer  series  of  hydrogen,  the  quantity  out- 
side the  brackets  representing  the  Rydberg  constant.  In 
fact,  as  a  splendid  confirmation  of  Bohr's  theory,  the  Ryd- 
berg constant,  N,  calculated  from  Millikan's  data,  furnishes 
a  value  which  agrees  with  that  found  from  spectral  series 
within  an  accuracy  of  one  part  in  1000. 


FIG.  4    Orbits  of  the  hydrogen  atom. 

The  successive  orbits  of  hydrogen  are  in  the  ratios  r,  22, 
32.  .  .  In  the  normal  state  of  the  atom  the  single  electron 
revolves  in  the  innermost  orbit.  When  the  atom  is  disturbed 


246  ECLIPSES    OF    THE    SUN 

ionized  helium  have  been  measured  by  Plaskett  in  certain 
O-type  stars.  At  the  position  of  HOL  the  difference  in  wave- 
length from  the  hydrogen  to  the  helium  line  is  2.63  A,  while 
at  H6  this  diminishes  to  1.54  A. 

It  has  thus  been  found  that  the  enhanced  spectrum  of 
helium  resembles  that  of  neutral  hydrogen,  and  in  exactly 
similar  manner  it  has  been  concluded  that  in  all  details  (even 
in  showing  doublets  or  triplets  in  their  spectra)  the  enhanced 
spectrum  of  an  alkali  earth  (like  Mg)  resembles  the  arc 
spectrum  of  the  alkali  metal  of  next  lower  atomic  number 
(like  No).  It  has  further  been  concluded  that  this  relation 
exists  for  all  alkali  earths  and  alkali  metals.  In  fact,  this 
similarity  in  spectra  between  neighboring  elements  in  the 
periodic  table  appears  to  be  a  general  rule,  namely,  that  the 
enhanced  spectrum  of  any  element  resembles  the  arc  spectrum 
of  the  element  of  next  lower  atomic  number.  If  an  element 
loses  two  electrons,  and  is  therefore  doubly  ionized,  its  spec- 
trum, for  the  same  reason,  should  be  similar  to  the  arc  spec- 
trum of  the  element  of  second  lower  atomic  number.  Fowler 
has  succeeded  experimentally  in  discovering  doubly-  and 
also  trebly-ionized  silicon.  Interesting  comparisons  have 
been  made  by  him  between  the  spectra  of  trebly-ionized  Si, 
doubly-ionized  Al,  singly-ionized  Mg  and  neutral  Na,  i.e., 
between  the  spectra  of  four  succeeding  elements  in  the  pe- 
riodic table  of  page  306.  In  the  laboratory  Millikan  almost 
at  will  has  been  able  to  strip  external  electrons  from  atoms. 

Although  the  mathematical  analysis  for  a  generalized 
theory  has  been  too  difficult  to  follow  through  to  completion, 
nevertheless  the  Bohr  theory  has  been  successful  in  placing 
spectrum  analysis  on  a  very  firm  foundation.  Further  de- 
velopments will  be  treated  in  the  following  chapters. 


CHAPTER  XV 

PHOTOGRAPHING  THE  FLASH  SPECTRUM 

HALF  a  century  of  remarkable  progress  in  solar 
physics  since  Young  first  observed  the  flash  spectrum 
at  the  eclipse  of  1870  saw  a  vast  diversity  of  opinion 
regarding  conditions  of  temperature  and  pressure  in  the 
envelope  causing  the  dark  Fraunhofer  lines.  On  the  one 
hand  appeared  Young's  original  view  that  the  reversing 
layer  is  a  thin  shell  but  a  few  hundred  miles  in  thickness, 
while  on  the  other  extreme  we  had  Lockyer's  opinion  that 
such  a  reversing  layer  does  not  exist  and  that  the  corona  is 
merely  the  cooler  and  rarer  portion  of  the  chromosphere. 
Within  the  past  decade,  however,  the  enormous  increase  in 
knowledge  concerning  the  structure  of  the  atom,  explained 
in  the  preceding  chapter,  coupled  with  the  publications  of 
discussions  of  the  spectrum  of  the  chromosphere  obtained  at 
solar  eclipses,  has  resulted  in  the  solution  of  many  of  the 
outstanding  problems  relating  to  the  sun. 

In  spite  of  the  enormous  improvement  in  recent  years  in 
instrumental  equipment  and  in  technique,  it  is  still  no  exag- 
geration to  state1  that  it  is  more  difficult  to  secure  a  per- 
fectly successful  photograph  of  the  spectrum  of  the  chromo- 
sphere than  it  is  to  obtain  an  excellent  photograph  of  any 
other  single  phenomenon  attacked  by  astrophysical  science. 
Witness  the  fact  that  since  its  first  observation  in  1870,  the 
flash  spectrum  has  continually  been  and  still  remains  one  of 
the  most  important  of  all  problems  taken  up  for  solution  at 
each  succeeding  solar  eclipse.  It  is  now  nearly  forty  years 
since  the  first  photograph  of  the  flash  spectrum  was  ob- 
tained, but  in  this  interval  of  time,  comparatively  long  when 
judged  by  the  attainments  of  modern  science,  there  have 

1  Cf.  Handbuch  der  Astrophysik,  4,  275,  1929. 
247 


248  ECLIPSES    OF    THE    SUN 

been  more  than  one  hundred  attempts  to  photograph  the 
flash  spectrum;  and  yet  there  are  not  more  than  a  half 
dozen  photographs  which  may  be  considered  to  rank  as  first 
quality.  Of  course,  it  is  perfectly  true  that  many  of  the 
attempts  might  have  succeeded  had  it  not  been  for  clouds, 
or  thin  haze,  or  poor  atmospheric  conditions,  yet  the  fact 
remains  that  the  great  majority  of  those  photographs  which 
escaped  the  clouds  have  suffered  from  lack  of  perfect  defini- 
tion or  from  inaccurate  timing  of  the  exposures. 

At  an  eclipse,  the  flash  spectrum  may  be  observed  visually 
or  photographically,  either  with  or  without  slit,  and  by  means 
of  prisms  or  gratings.  The  researches  of  the  present  day 
demand  large  dispersion.  The  type  of  spectrograph  depends 
very  largely  on  the  particular  problem  which  the  eclipse  ob- 
server desires  to  solve.  The  great  advantage  of  the  prism 
is  the  greater  light-gathering  power,  the  light  being  concen- 
trated in  one  spectrum,  but  on  the  other  hand  the  grating 
possesses  many  points  in  its  favor.  The  lines  in  the  prismatic 
spectrum  are  crowded  together  at  the  red  end  and  widened 
out  at  the  blue  part  of  the  spectrum,  thus  entailing  much 
difficulty  in  the  determination  of  wave-lengths.  The  grating 
gives  a  normal  spectrum,  permits  of  higher  dispersion,  and 
gives  higher  resolving  power,  a  larger  extent  of  spectrum 
and  probably  better  definition.  Gratings  either  plane  or 
concave  may  be  used,  but  with  a  flat  grating  a  lens  becomes 
necessary  to  bring  the  spectrum  to  a  focus,  and  such  a  lens 
introduces  aberrations  and  absorption  of  light,  and  conse- 
quent loss  of  definition.  In  the  past,  plane  gratings  have 
been  used  at  eclipses,  but  for  the  future  it  is  safe  to  predict 
that  whenever  gratings  are  employed  for  the  chromosphere 
they  will  be  concave  gratings  and  not  plane. 

The  spectrograph  which  lends  itself  most  readily  to  the 
easiest  and  sharpest  focus  under  the  temporary  conditions 
of  eclipse  observations  is  undoubtedly  one  used  with  slit. 
For  this  purpose  it  is  possible  to  take  bodily  into  the  field  a 
spectroscope  of  the  type  that  has  been  thoroughly  tested 
and  tried  out  on  stars  in  the  regular  work  of  the  observatory. 
This  instrument  is  universally  a  prism  spectograph  with 
prisms  of  the  very  highest  quality.  It  is  readily  possible  to 


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PHOTOGRAPHING    THE    FLASH    SPECTRUM    249 

test  thoroughly  the  spectograph  at  home  and  then  to  mark 
carefully  the  positions  of  the  focus  for  collimator  and  camera 
before  dismounting.  It  is  the  work  of  a  short  while  to  again 
assemble  the  spectrograph  at  the  eclipse  site,  and  since  the 
whole  spectrograph  is  comparatively  light  in  weight  it  can 
easily  be  picked  up  and  set  down  anywhere.  For  instance, 
it  is  readily  possible  to  test  the  focus  thoroughly  by  the  use 
of  the  electric  arc,  or  by  taking  a  focus  plate  of  the  daylight 
sky.  At  the  time  of  the  eclipse  such  an  instrument  is  ordi- 
narily used  with  a  heliostat  or  coelostat  mirror,  and  with  the 
employment  of  a  lens  or  concave  mirror  for  forming  an  image 
of  the  sun  on  the  slit  of  the  spectrograph.  With  this  type 
of  spectrograph  an  eclipse  observer  should  be  absolutely 
certain  to  secure  spectra  of  the  chromosphere  in  excellent 
definition  at  each  and  every  eclipse  where  clouds  do  not 
interfere.  The  only  chances  of  failure  with  such  an  instru- 
ment are  poor  focus  of  the  spectrograph  and  inability  to 
keep  the  portion  of  the  image  of  the  sun,  selected  for  observa- 
tion, on  the  slit  at  the  time  of  the  eclipse.  Any  observer 
skilled  in  observatory  manipulation  through  long  years  of 
handling  a  slit  spectrograph  should  be  ashamed  to  admit 
that  the  focus  was  poor  due  to  the  lack  of  careful  methods 
of  adjustment.  There  is  however  some  excuse  if  a  skilled 
astronomer  fails  to  keep  the  slit  of  the  spectrograph  filled 
with  light  for  the  reason  that  the  few  moments  of  an  eclipse 
are  very  tense  and  excited,  the  light  at  totality  is  feeble,  and 
after  all,  even  an  efficient  observer  must  at  some  time  in  his 
career  see  his  first  total  eclipse  —  and  he  will  have  few  oppor- 
tunities of  witnessing  others. 

By  the  use  of  slit  spectrographs,  the  British  observers  have 
obtained  excellent  photographs  of  the  flash  spectrum.  At 
the  1926  eclipse  observed  in  Sumatra,  Davidson  and  Strat- 
ton *  used  a  quartz  spectrograph  designed  to  photograph  to 
the  extreme  limit  of  the  ultra-violet.  The  prism  train  con- 
sisted of  four  double  quartz  prisms  of  60°  angle,  each  prism 
being  composed  of  two  half-prisms  of  right-  and  left-handed 
quartz.  The  collimator  and  camera  objectives  were  each 
a  single  quartz  lens  of  3 -inches  aperture  and  36-inches  focus. 

1  Memoir  R.  A.  S.,  64,  105,  1927. 


250  ECLIPSES    OF    THE    SUN 

Excellent  definition  was  secured  between  wave-lengths  3066 
A  and  4200  A. 

The  greatest  dispersion  ever  used  at  an  eclipse,  before 
that  of  1932,  was  employed  by  Pannekoek  and  Minnaert1 
in  1927.  Their  optical  system  consisted  of  many  elements 
each  of  unusually  large  dimensions.  After  reflection  from 
the  coelostat  mirror,  the  rays  passed  first  through  an  image 
lens  10  inches  in  diameter.  After  passing  through  a  slit,  a 
totally  reflecting  prism  turned  the  beam  through  90°,  while 
next  in  the  train  came  a  triple  collimating  lens  of  6-inch 
diameter.  After  traversing  three  prisms  of  45°  angles  and 
large  enough  to  receive  the  full  beam  of  light,  the  rays  were 
reflected  from  a  plane  mirror,  and  then  they  passed  in  re- 
versed order  through  the  prisms  and  6-inch  objective,  which 
now  became  the  camera  lens  for  forming  an  image  on  the 
photographic  plate.  Good  definition  was  obtained  in  the 
region  of  wave-lengths  4153  to  4751  A. 

Although  it  is  easier  to  photograph  the  flash  spectrum  by 
using  a  slit,  nevertheless  a  slit  is  entirely  unnecessary,  the 
crescent  arcs  of  the  chromosphere  providing  narrow  curved 
lines  in  the  photographed  spectrum.  Good  reasons  for  dis- 
carding the  slit  will  be  given  later.  However,  it  is  a  much 
more  difficult  task  to  secure  perfect  focus  of  the  chromo- 
spheric  spectrum  without  a  slit,  either  by  the  use  of  objective 
prisms  or  grating.  Slitless  spectrographs  of  the  power  essen- 
tial for  the  flash  spectrum  are  practically  never  used  in 
observatory  or  laboratory.  In  stellar  work,  a  prism  is  placed 
in  front  of  the  objective,  forming  what  is  universally  called 
a  "  prismatic  camera. "  For  many  long  years  this  type  of 
instrument  has  been  used  in  routine  observatory  work,  but 
ordinarily  the  dispersion  is  small  and  therefore  unsuited  to 
making  any  contribution  to  our  present  knowledge  of  the 
flash  spectrum.  The  dispersion  of  such  a  combination  may 
be  increased  by  two  different  methods:  (i)  by  increasing 
the  focal  length  of  the  objective;  or  (2)  by  increasing  the 
number  of  prisms.  At  the  1926  eclipse,  Davidson  and  Strat- 
ton  used  an  ingenious  device,  the  camera  of  ig-foot  focus 
being  utilized  in  a  dual  capacity.  With  a  direct- vision  slipped 

1  Verh.  Kon.  Akad.  ie  Amsterdam,  13,  No.  5,  1928. 


PHOTOGRAPHING    THE    FLASH    SPECTRUM     251 

in  front  of  the  objective  the  flash  spectrum  was  photographed, 
while  with  the  prism  removed,  direct  photographs  of  the 
corona  were  obtained.  Even  with  the  moderate  focal  length 
of  19  feet,  necessary  to  secure  the  desired  dispersion,  the 
crescent  arcs  were  two  inches  in  diameter  on  the  photo- 
graphs. As  will  be  explained  later,  the  average  conditions 
of  seeing  experienced  at  eclipses  may  cause  these  crescents 
forming  the  lines  of  the  spectrum  to  lose  their  clear-cut  char- 
acter in  spite  of  the  fact  that  the  focus  of  the  instrument 
may  be  perfect.  None  of  the  half  dozen  photographs  of  the 
flash  spectrum  which  the  writer  regards  as  of  the  highest 
class  of  excellence  have  been  obtained  by  this  type  of  in- 
strument, where  the  increased  focal  length  of  the  camera  has 
been  relied  upon  to  increase  the  dispersion. 

On  the  other  hand,  excellent  spectra  have  been  obtained 
by  objective  prisms  without  the  use  of  slit,  through  the  em- 
ployment of  additional  prisms  to  increase  the  dispersion. 
At  the  eclipse  of  1905  in  Spain,  Campbell 1  secured  exquisite 
definition  with  two  60°  prisms  in  front  of  an  objective  of 
2 -inch  aperture  and  6o-inch  focus.  Lines  were  recorded 
between  wave-lengths  3820  and  5300  A,  though  the  defini- 
tion began  to  fail  in  the  neighborhood  of  4650  A.  The  dis- 
persion at  Hy  was  5.7  A  per  mm.  A  moving  plate  was  used 
in  forming  the  spectra,  a  method  which  will  be  explained 
later.  At  the  Flint  Island  eclipse  of  1908,  Campbell  in- 
creased the  dispersion  by  the  use  of  three  60°  prisms  and 
an  objective  of  60  inches  focal  length,  the  dispersion  being 
about  double  that  obtained  by  him  in  1905.  At  Hy  this 
amounted  to  2.9  A  per  mm,  almost  as  great  in  scale  as  in  the 
1927  spectra  of  Pannekoek  and  Minnaert. 

In  the  observation  of  eclipses,  my  own  work  has  always 
been  spectroscopic.  I  have  always  employed  gratings  and 
have  used  them  without  a  slit.  My  record  as  an  eclipse  ob- 
server is  probably  unique.  I  have  observed  eight  solar  eclip- 
ses, and  yet  neither  the  Leander  McCormick  Observatory 
nor  I,  personally,  owns  a  single  piece  of  eclipse  apparatus; 
at  each  eclipse  everything  must  be  borrowed.  At  my  latest 
eclipse,  in  1930  at  Niuafoou  Island,  I  had  two  concave  grat- 

1  Publications  of  the  Lick  Observatory,  17,  i,  1931. 


252  ECLIPSES    OF    THE    SUN 

ings.  All  of  the  equipment  used  at  this  expedition  belonged 
to  the  U.  S.  Naval  Observatory  with  the  exception  of  one 
grating  of  4-inch  aperture  kindly  loaned  by  my  good  friend 
Saunders  of  Harvard  University.  This  grating  has  been 
with  me  at  every  eclipse  since  1905.  Photographs  of  the 
flash  spectrum  were  obtained  in  all  but  two  of  the  eclipses, 
in  1923  in  California  and  in  1927  in  Norway.  The  results 
from  the  photographs  obtained  under  clear  skies  in  1905 
and  1925  have  been  published  in  Astrophysical  Journal,  71 , 
i,  1930,  and  72,  146,  1930.  The  other  concave  grating  used 
in  1930  was  of  6-inch  aperture.  Both  gratings  are  of  lo-foot 
radius  of  curvature  (or  6o-inch  focus)  and  each  has  approxi- 
mately 15,000  lines  per  inch.  The  dispersion  is  10.9  A 
per  mm.  At  this  recent  eclipse,  excellent  definition  was 
obtained  from  3200  in  the  violet  to  7800  A  in  the  red, 
the  region  4650  to  6800  A  being  photographed  by  both 
spectrographs.  The  discussion  of  these  spectra  has  not  been 
completed.  The  same  two  gratings  will  be  employed  at  the 
1932  eclipse. 

The  arrangement  for  using  the  concave  grating  without  a 
slit  is  one  of  the  greatest  simplicity.  The  light  from  the  sun 
falls  directly  on  the  coelostat  mirror  and  then  after  reflec- 
tion the  beam  of  parallel  light  falls  on  the  grating  where  it  is 
diffracted  and  brought  to  a  focus  on  the  photographic  plate. 
If  the  grating  and  the  photographic  plate  are  each  perpen- 
dicular to  the  line  joining  their  centers,  the  spectrum  is  nor- 
mal, or  to  speak  in  more  exact  terms,  the  spectrum  departs 
very  little  from  a  uniform  scale  of  wave-lengths. 

Used  in  the  ordinary  Rowland  form  of  mounting  in  the 
laboratory,  one  of  the  well  recognized  advantages  of  the 
concave  grating  is  the  property  of  "  astigmatism/'  whereby 
the  spectrum  lines  are  increased  in  length.  If  the  astigma- 
tism should  be  of  approximately  the  same  amount  when  the 
grating  is  used  objectively  without  slit,  then  as  a  result  of 
lengthening  out  the  chromospheric  lines,  which  are  neces- 
sarily curved,  the  definition  would  be  ruined.  Consequently, 
in  making  plans  for  1900,  when  concave  gratings  were  used 
for  the  first  time  at  an  eclipse,  the  Naval  Observatory  party 
did  not  dare  attempt  to  use  such  a  grating  without  slit. 


PHOTOGRAPHING   THE    FLASH    SPECTRUM    253 

The  successful  photographs  of  stellar  spectra  l  secured  by 
concave  grating  used  objectively  showed  however  that  these 
fears  were  groundless.  Moreover,  Runge's  discussion  of  the 
theory  of  the  concave  grating,  in  Kayser's  Handbuch  der 
Spectroscopie,  i,  450,  1900,  proved  that  the  amount  of  as- 
tigmatism for  a  concave  grating  used  in  the  objective  form 
would  be  so  minute  that  it  could  have  no  harmful  effects 
on  the  definition  of  the  spectra. 

It  should  hardly  be  necessary  to  add  that  the  grating 
spectroscope  must  be  very  firmly  mounted  on  solid  piers  of 
masonry  or  heavy  timbers  in  order  that  the  tremors  of  the 
apparatus  caused  by  the  wind  or  by  the  changing  of  the 
plate  holders  may  quickly  subside.  It  is  manifestly  difficult 
to  mount  such  an  instrument  of  large  dispersion  on  an  equa- 
torial mounting  or  on  a  polar  axis,  with  the  grating  in  conse- 
quence directly  exposed  to  the  sun's  rays.  This  method 
would  indeed  get  rid  of  the  coelostat  mirror  with  its  possible 
change  in  figure,  but  if  this  plan  were  followed,  it  would 
probably  be  a  case  of  "  out  of  the  frying  pan  into  the  fire." 

It  might  not  be  out  of  place  to  call  attention  to  the  very 
great  difficulty  always  experienced  by  eclipse  observers  in 
securing  sharp  focus  with  their  spectroscopes;  as  stated,  one 
very  prominent  feature  of  eclipse  spectra  in  the  past  has 
been  the  continued  succession  of  photographs  poorly  fo- 
cused. One  method  of  securing  focus  frequently  made  use 
of  has  been  to  apply  the  final  adjustments  by  utilizing  the 
spectrum  of  the  disappearing  crescent  of  the  sun  a  few 
minutes  before  totality.  During  the  excitement  and  nerve- 
racking  tension  of  these  moments,  a  perfect  adjustment  can 
be  obtained  in  this  manner  only  as  the  result  of  a  happy  acci- 
dent —  and  this  method  should  never  be  resorted  to  under 
any  circumstances.  For  instruments  of  small  dispersion,  the 
light  from  a  star  may  be  utilized  for  securing  focus,  if  hap- 
pily some  bright  star  is  conveniently  located,  but  for  spectro- 
graphs  of  the  greatest  dispersion  the  spectra  even  from  the 
brightest  of  the  stars  are  too  weak.  For  large  instruments, 
there  is  left  only  the  option  of  securing  focus  on  the  sun  itself 
or  the  electric  arc  several  days  before  the  eclipse  by  the  em- 

1  Astrophysical  Journal,  10,  29,  1900. 


254  ECLIPSES    OF    THE    SUN 

ployment  of  some  sort  of  collimating  device  which  will  give 
a  parallel  beam  of  light  coming  from  a  slit  source.  For  the 
use  of  the  U.  S.  Naval  Observatory  party  at  the  eclipse  of 
1905,  Jewell  constructed  a  collimator  consisting  of  a  slit  at 
the  common  focus  of  two  concave  mirrors,  lenses  not  being 
used  because  of  their  chromatic  aberrations.  Several  meth- 
ods of  placing  the  slit  at  the  common  focus  of  the  mirrors 
will  at  once  suggest  themselves  to  any  ingenious  eclipse  ob- 
server, one  of  the  simplest  being  to  utilize  a  telescope  of  me- 
dium size  (say  of  five  inches  aperture)  accurately  focused  on 
the  stars. 

The  author  has  always  made  it  a  habit  to  be  at  the  eclipse 
site  a  month  or  more  in  advance  so  that  there  may  be  plenty 
of  time  to  secure  exact  focus  without  too  much  rush  and 
excitement  immediately  preceding  the  eclipse.  The  collima- 
tor described  above  has  always  been  utilized.  When  electric 
power  is  available,  it  has  been  found  easier  and  more  efficient 
to  use  the  electric  arc  as  the  source  of  light  for  adjusting 
rather  than  the  sun.  This  method  was  followed  in  the 
eclipses  of  1923,  1925  and  1927.  At  the  "  Tin-Can  Island  " 
eclipse  there  was  no  electric  power,  and  hence  there  was  noth- 
ing to  do  but  use  the  tropical  sun  as  the  source  of  light  during 
the  weeks  of  adjustment.  The  difficulties  underlying  this 
process  are  at  once  evident.  In  the  optical  train  while  ad- 
justing, there  are  four  mirror  surfaces:  the  coelostat  of 
silver-on-glass,  the  two  metal  mirrors  of  the  collimator  and 
the  grating  itself.  In  the  adjustment,  the  slit  becomes  the 
source  of  light,  and  hence  it  is  of  minor  importance  if  the 
coelostat  mirror  is  not  plane  or  if  the  slit  is  not  in  the  exact 
focus  of  the  first  collimator  mirror  which  forms  the  image 
of  the  sun  on  the  slit. 

Unfortunately,  mirrors  alter  their  focal  lengths  with 
changes  in  temperature,  a  fact  well  known  to  every  astrono- 
mer with  experience  with  a  reflecting  telescope.  Excellent 
focus  of  the  flash  spectrum  requires,  among  other  things,  that 
on  the  eclipse  day  the  coelostat  mirror  be  plane  and  not 
warped  by  the  direct  rays  of  the  sun.  (The  mirror  is  always 
screened  from  sunlight  until  a  few  minutes  before  totality.) 
No  matter  how  much  time  has  been  spent  nor  how  much  care 


PHOTOGRAPHING    THE    FLASH    SPECTRUM     255 

in  the  determination  of  focus,  the  great  fear  is  always  present 
lest  the  grating  alter  its  focal  length  owing  to  changes  of 
temperature  between  the  time  of  adjustment  and  the  time 
of  the  eclipse.  This  requires  that  the  wooden  boxes  and 
plate  holders  of  the  spectrograph  should  not  expand  enough 
to  deteriorate  the  focus.  And  all  of  these  nice  adjustments 
had  to  be  made  and  relied  upon  at  the  recent  eclipse  on  a 
tropical  island,  with  clouds  always  interfering  with  the  work, 
with  the  dark-room  during  the  daytime  with  temperatures 
that  felt  like  inferno,  and  with  no  ice  and  no  running  water 
for  development  of  the  photographs.  The  trials  and  tribu- 
lations of  an  eclipse  astronomer  are  many,  especially  when 
it  is  necessary  to  work  with  apparatus  which  is  temporary 
and  always  borrowed ! 

With  the  collimator  placed  in  the  beam  of  light  between 
coelostat  mirror  and  grating,  the  focus  was  first  determined 
visually  with  as  great  care  as  possible.  For  improving  the 
focus,  about  a  hundred  and  fifty  focus  plates  were  taken. 
The  extreme  range  of  focus  in  the  last  hundred  of  these  films 
was  but  a  fiftieth  of  an  inch  together  with  slight  changes  in 
the  tilt  of  the  photographic  film. 

The  best  type  of  spectograph  to  plan  for  the  photography 
of  the  flash  spectrum  will  unquestionably  be  the  one  that  is 
able  to  make  the  greatest  contribution  to  the  solution  of  solar 
problems.  With  the  gigantic  equipment  of  great  observa- 
tories working  assiduously  on  the  dark-line  absorption  spec- 
trum of  the  sun,  with  the  knowledge  of  the  structure  of  the 
atom  gleaned  in  recent  years  from  refined  researches  in  ob- 
servatory and  laboratory,  how  can  the  bright-line  emission 
soectrum  of  the  sun  photographed  with  comparatively  sm^ll 
dispersion  best  contribute  to  these  investigations  by  supple- 
menting daily  researches?  The  chromospheric  soectra  ?*re 
investigated  in  order  to  ascertain:  (i)  wave-lengths;  (2)  in- 
tensities of  the  lines;  and  (3)  the  heights  in  miles  or  kilo- 
meters that  the  vapors  extend  above  the  photosphere. 

It  will  be  shown  later  that  the  intensities  of  the  lines  of  the 
flash  spectrum  depend  on  a  great  many  different  considera- 
tions, the  timing  of  the  exposures,  the  heights  and  whether 
the  spectrograph  is  with  slit  or  slitless.  For  refined  work 


256  ECLIPSES    OF    THE    SUN 

where  the  intensities  are  measured  by  a  microphotometer,  all 
of  the  various  factors  must  be  taken  into  account.  If,  how- 
ever, the  intensities  are  estimated  on  an  arbitrary  scale  in 
the  manner  of  Rowland's  Tables,  these  estimates  may  be 
carried  out  with  about  equal  facility  no  matter  what  the 
type  of  instrument.  Consequently,  in  making  a  decision 
regarding  the  best  type  of  spectrograph,  the  question  of  in- 
tensities may  be  laid  to  one  side.  One  has  a  choice  that 
is  accordingly  much  limited.  Are  exact  wave-lengths  of  the 
chromosphere  of  the  highest  importance? 

If  the  decision  is  made  in  the  affirmative,  then  it  might 
seem  that  we  should  adopt  a  slit  spectrograph.  With  equal 
dispersion  and  equally  sharp  focus,  the  wave-lengths  derived 
from  a  slit  spectrograph  will  however  be  superior  in  accuracy 
to  those  with  slitless  instruments  in  the  measurement  of  the 
strongest  lines  of  the  spectrum  only.  If  the  observer  is  care- 
ful to  place  the  slitless  spectrograph  in  such  a  position  that  the 
tangents  at  the  centers  of  the  chromospheric  arcs  lie  parallel 
to  the  lines  of  the  grating  or  to  the  edge  of  the  prism,  or  in 
other  words,  so  that  the  spectral  lines  are  perpendicular  to 
the  length  of  the  spectrum,  then  these  spectra  are  sharp  and 
clear-cut  and  wave-lengths  can  be  determined  with  high  pre- 
cision. With  the  strongest  lines  in  all  spectra  there  is  always 
a  spreading  of  light  on  the  photographic  plate  by  irradiation. 
If  a  slit  is  employed  this  spreading  is  fairly  symmetrical, 
while  without  a  slit  the  spreading  is  not  symmetrical  due  to 
the  interposition  of  the  moon  and  to  the  elevation  of  the 
heated  vapors  above  the  surface  of  the  photosphere.  Slitless 
spectra  of  the  chromosphere  consequently  demand  greater 
experience  and  judgment  in  the  person  who  measures  these 
spectra  with  the  result  that  the  precision  of  the  wave-lengths 
will  depend  to  a  large  measure  on  the  skill  of  the  measurer 
in  his  ability  to  allow  for  the  effects  of  irradiation. 

With  the  small  amount  of  light  available  at  eclipses  and 
brief  exposures  of  a  few  seconds,  it  is  possible  only  to  utilize 
a  dispersion  small  in  size  when  compared  with  the  much 
greater  dispersion  in  the  every-day  solar  investigations,  such 
as  are  carried  out  with  the  i5q-foot  tower  telescope  and  75- 
foot  spectrograph  of  the  Mount  Wilson  Observatory.  A 


PHOTOGRAPHING   THE    FLASH    SPECTRUM    257 

dispersion  of  about  2  or  3  angstroms  to  the  millimeter  is 
about  the  maximum  possible  with  the  flash  spectrum  and  this 
permits  an  accuracy  of  wave-lengths  of  about  0.02  angstroms. 
Such  an  accuracy  is  much  inferior  to  that  employed  in  ordi- 
nary solar  work,  and  is  not  sufficiently  high  to  permit  the 
determination  of  systematic  differences  between  eclipse 
wave-lengths  and  those  taken  under  ordinary  solar  condi- 
tions. Accordingly,  chromospheric  wave-lengths  practi- 
cally can  serve  no  other  purpose  than  the  identification  of 
lines  for  accurate  comparisons  with  Rowland's  Tables  in 
order  to  determine  the  sources  whence  the  spectral  lines 
originate. 

As  a  matter  of  fact,  no  photographs  of  the  flash  spectrum 
taken  with  a  slit  have  furnished  any  more  accurate  wave- 
lengths than  those  of  the  1905  eclipse  taken  by  concave 
grating  without  slit.  It  therefore  appears  open  to  question 
whether  spectrographs  with  slits  will  give  wave-lengths  of 
greater  accuracy  than  those  of  the  slitless  variety. 

If,  therefore,  we  assume  that  both  wave-lengths  and  in- 
tensities of  the  spectral  lines  may  be  determined  with  equal 
accuracy  with  slit  and  slitless  instruments  of  about  the  same 
dispersion,  the  choice  of  the  best  type  of  spectrograph  for 
the  flash  spectrum  must  be  decided  by  the  ability  of  the 
two  different  types  of  instruments  to  give  information  re- 
garding the  heights  to  which  the  chromospheric  vapors  extend 
above  the  surface  of  the  photosphere.  The  decision  can 
now  be  made  in  the  easiest  possible  manner.  Spectrographs 
without  slits  give  vastly  more  information  about  levels  than 
can  possibly  be  derived  from  any  form  of  slit  instrument. 

It  is,  accordingly,  the  firm  opinion  of  the  writer,  who  has 
confined  his  attention  at  eclipses  exclusively  to  the  photog- 
raphy of  the  flash  spectrum,  that  an  observer  who  chooses  a 
slit  spectrograph  to  photograph  the  flash  spectrum  delib- 
erately thrusts  to  one  side  the  most  valuable  information  that 
can  come  from  the  chromospheric  spectrum,  namely,  the 
heights  to  which  the  vapors  extend  above  the  sun's  surface. 
On  the  other  hand,  it  is  well  not  to  forget  that  a  spectrograph 
with  slit  readily  permits  a  more  accurate  and  reliable  de- 
termination of  good  focus  under  the  temporary  conditions  of 


258  ECLIPSES    OF    THE    SUN 

eclipse  observation  than  is  possible  with  any  form  of  slitless 
instrument.  It  would  therefore  perhaps  be  good  advice  to 
say  that  if  an  astronomer  is  to  observe  his  first  eclipse  and 
wishes  to  do  some  work  of  value,  he  had  better  try  to  photo- 
graph the  flash  spectrum  using  a  slit  spectrograph  with  fair 
dispersion,  for  instance,  about  equal  to  that  of  the  Mills 
spectrograph  of  the  Lick  Observatory  or  the  Bruce  of  the 
Yerkes  Observatory.  For  such  work  it  will  be  well  for  him  to 
confine  his  attention  to  the  blue  end  of  the  spectrum  on  ac- 
count of  the  greater  dispersion  of  his  spectrograph  in  this 
region  and  on  account  of  the  greater  number  of  lines  in  the 
flash  spectrum. 

It  is  impossible  to  exaggerate  the  importance  of  securing 
the  photographs  of  the  flash  spectrum  at  the  proper  instants 
of  time  so  as  to  secure  the  spectra  of  the  layers  of  the  chromo- 
sphere as  close  to  the  photosphere  as  possible.  At  an  eclipse, 
there  are  two  manifestations  of  the  flash,  one  at  the  begin- 
ning and  one  at  the  end  of  totality.  Before  the  begin- 
ning of  the  total  eclipse,  the  Fraunhofer  lines  persist  as  long 
as  there  is  any  portion  of  the  photosphere  visible,  but  when 
the  moon  entirely  covers  the  sun's  surface,  or  at  the  very  in- 
stant of  the  beginning  of  totality,  there  is  the  sudden  re- 
versal of  the  Fraunhofer  spectrum  to  that  of  bright  lines. 
If  one  watches  the  phenomenon  visually  with  some  form  of 
spectroscope,  he  will  see  many  of  the  high  level  lines  reversed 
many  minutes  before  totality,  particularly  at  the  cusps. 

The  plan  for  securing  photographs  at  the  proper  times  al- 
ways followed  by  the  author  is  an  old  familiar  one.  A  pair 
of  old-fashioned  binoculars  is  used  and  over  one  lens  there  is 
a  direct-vision  spectroscope.  The  one  used  in  recent  eclipses 
is  a  replica  transmission  grating  of  15,000  lines  per  inch. 
With  this  it  is  possible  to  observe  any  particular  portion  of 
the  spectrum  one  wishes.  With  a  pair  of  binoculars  and  such 
an  attachment,  it  is  possible  with  the  left  eye  to  watch  the 
disappearing  crescent  of  the  sun,  shielding  the  eye  by  smoked 
or  colored  glass,  while  with  the  right  eye  the  emission  lines 
can  be  watched  as  they  appear  one  after  the  other  with  the 
approach  of  second  contact.  Armed  with  this,  the  first  flash 
can  be  observed  and  the  exposure  started  with  great  nicety. 


PHOTOGRAPHING    THE    FLASH    SPECTRUM    259 

On  account  of  the  uncertainty  in  our  knowledge  of  the  exact 
duration  of  totality  still  persisting,  one  will  be  wise  to  de- 
cide that  the  exposure  for  the  second  flash  should  begin  five 
seconds  before  the  calculated  end  of  totality,  and  should  ter- 
minate with  the  first  trace  of  the  reappearing  sun.  A  delay 
of  one-tenth  of  a  second  in  ending  the  exposure  may  readily 
bring  ruin  to  this  exposure.  In  photographing  the  flash  spec- 
trum at  an  eclipse  it  is  evident  that  the  important  photo- 
graphs are  two,  one  at  the  beginning  and  one  at  the  end  of 
totality.  Ordinarily,  additional  photographs  are  made,  just 
before  and  immediately  after  totality,  for  the  Fraunhofer 
and  any  emission  lines.  During  totality,  several  short  ex- 
posures are  given,  just  after  the  first  flash  and  again  before 
the  second  flash,  for  the  vapors  of  greater  elevation,  with  a 
long  exposure  or  two  at  mid-totality  to  obtain  the  spectrum 
of  the  corona  which  appears  as  a  series  of  complete  rings. 

My  little  pair  of  binoculars  just  described  gave  me  at  the 
1930  eclipse  a  most  gratifying  view  of  the  flash  spectrum, 
both  at  the  beginning  and  at  the  end  of  totality.  One  great 
handicap  in  eclipse  work  is  that  in  one's  lifetime  there  are  so 
few  opportunities  to  rehearse.  The  1930  photographs  show 
that  my  reaction  time  (like  that  of  everybody  else  who 
has  ever  photographed  the  flash  spectrum )  is  not  instantane- 
ous; the  first  flash  does  not  record  the  chromospheric  lines 
of  lowest  level.  At  the  end  of  totality,  I  kept  the  exposure 
going  as  long  as  I  dared,  and  the  photograph  shows  almost 
perfect  timing. 

It  need  hardly  be  added  that  the  times  of  first  and  second 
flash,  recorded  preferably  on  the  chronograph,  will  furnish 
excellent  observations  of  the  beginning  and  ending  of  the 
total  eclipse.  On  account  of  the  rough  character  of  the 
moon's  edge,  these  times  will  not  agree  with  those  recorded 
from  the  last  appearance  and  first  reappearance  of  Baily's 
beads  —  which  brings  up  anew  the  time-honored  question  of 
what  one  means  by  the  "  beginning  "  and  "  ending  "  of  a 
total  eclipse. 

The  above  descriptions  of  slitless  instruments  and  meth- 
ods of  obtaining  the  flash  spectrum  all  refer  to  a  stationary 
photographic  plate  where  the  lines  of  the  spectrum  consist  of 


260 


ECLIPSES    OF    THE    SUN 


a  series  of  crescents.  At  the  eclipse  of  1898  in  India,  Camp- 
bell tried  out  an  ingenious  innovation  of  placing  a  narrow 
slot  directly  in  front  of  the  photographic  plate  in  the  manner 
shown  in  Figure  5 ;  and  then  moving  the  photographic  plate 
uniformly  in  a  direction  perpendicular  to  the  slot.  The  same 
procedure  was  followed  at  the  eclipse  of  1900  in  Georgia, 
1905  in  Spain  and  1908  in  Flint  Island.  The  best  definition 
of  the  four  eclipses  was  obtained  in  1905.  After  a  delay  of 


FIG.  5    The  moving-plate  spectrograph 


more  than  twenty  years,  a  discussion  by  Menzel,  together 
with  an  introduction  by  Campbell,  has  recently  appeared  in 
handsome  form  as  Volume  XVII  of  the  Publications  of  the 
Lick  Observatory.  This  volume  forms  a  magnificent  con- 
tribution to  the  study  of  solar  problems,  quite  in  keeping  with 
the  high  standard  of  excellence  set  in  the  past  forty  years  in 
the  observation  of  eclipses  by  the  Lick  Observatory  and  by 
Dr.  Campbell,  its  director  for  much  of  this  period.  The 
author  wishes  to  express  his  highest  praise  of  the  beautiful 
Lick  spectra  and  of  the  splendid  character  of  the  discussion. 
If  the  moving  plate  is  set  in  motion  about  twenty  seconds 
before  the  beginning  of  totality,  it  will  automatically  record 
the  gradual  appearance  as  bright  lines,  first  of  the  lines  of 
highest  level,  then  of  medium  level  lines  and  then  the  spec- 
tacular appearance  of  the  flash  spectrum  in  the  manner 
described  by  Young  at  the  eclipse  of  1870.  At  the  end  of 
totality  the  plate  should  be  started  more  than  five  seconds 
before  the  calculated  end  of  totality  in  order  to  photograph 
the  second  appearance  of  the  flash  spectrum  in  reversed  or- 


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PHOTOGRAPHING    THE    FLASH    SPECTRUM    261 

der.  Facing  page  260,  there  is  a  reproduction  of  the  Lick 
moving  plate  of  the  second  flash  of  the  1905  eclipse.  The 
strip  of  normal  Fraunhofer  spectrum  of  the  sun  appended 
below  for  comparison  was  taken  by  Menzel  with  the  original 
1905  apparatus  reassembled  after  more  than  twenty  years 
and  with  the  addition  of  a  slit  and  collimator  lens.  The 
streaks  running  lengthwise  throughout  the  spectrum  are 
caused  by  the  non-uniform  motion  of  the  photographic  plate. 

It  is  interesting  to  compare  the  details  of  this  moving  plate 
spectrum  with  that  taken  by  the  author  with  fixed  plate,  as 
shown  facing  page  248.  The  spectra  taken  by  the  two 
methods  refer  to  the  same  (1905)  eclipse  and  both  were 
taken  at  the  end  of  totality.  In  the  original  spectra,  in  the 
region  near  the  strong  H  and  K  lines  shown  to  the  left  on 
page  248,  the  Lick  moving  plate  had  three  times  the  disper- 
sion of  the  fixed  Mitchell  plate. 

One  obvious  and  important  advantage  of  the  moving  over 
the  fixed  plate  is  that  the  former  makes  less  drastic  demands 
on  the  ability  of  the  observer  to  time  his  exposures  so  as  to 
reach  the  lowest  possible  levels.  Another  advantage  is  that 
the  moving  plate  records  the  gradual  changes  from  the  dark- 
to  the  bright-line  spectrum  at  first  flash,  and  in  the  reversed 
order  at  second  flash.  The  fixed  plate  makes  the  record  in 
discontinuous  exposures,  the  moving  plate  has  a  continuous 
record. 

Now  that  the  Lick  results  have  been  published,  it  is  pos- 
sible to  compare  the  relative  merits  of  the  fixed  and  moving 
plates  for  the  photography  of  the  flash  spectrum.  When 
the  performances  of  any  two  spectrographs  are  compared,  it 
is  almost  the  universal  custom  among  skilled  spectroscopists 
to  draw  conclusions  only  when  the  two  instruments  have 
nearly  identical  optical  powers,  but  more  particularly  when 
the  dispersion  is  nearly  the  same  for  each.  By  disregarding 
this  basis  of  comparison,  many  mistakes  have  been  made  in 
interpreting  the  results  of  the  flash  spectrum,  the  most  out- 
standing probably  being  that  by  Lockyer,  referred  to  on  page 
162,  who  compared  the  number  of  lines  in  the  flash  spectra 
photographed  in  1893  and  1896  with  the  number  found  in  the 
great  Rowland  Atlas  obtained  with  vastly  superior  definition 


262  ECLIPSES  OF  THE  SUN 

and  dispersion.  One  is  therefore  surprised  to  find  Campbell 
(loc.  cit.)  comparing  the  number  of  spectral  lines  of  the 
1905  and  1908  moving  plates  with  those  given  by  the  1905 
fixed  plate  with  concave  grating,  and  then  drawing  conclu- 
sions in  spite  of  the  fact  that  the  1905  moving  plate  had  twice 
the  dispersion,  and  the  1908  plate  four  times  the  dispersion 
in  the  region  of  wave-lengths  common  with  the  concave  grat- 
ing photograph. 

Investigations  of  the  flash  spectrum  are  attempted  in  order 
to  photograph  the  solar  spectrum  under  conditions  differing 
radically  from  those  of  every-day  observation  without  an 
eclipse.  The  heights  to  which  chromospheric  vapors  extend 
are  of  the  very  greatest  value  in  furthering  solar  research. 
Hence,  if  comparisons  are  made  of  fixed  and  moving  plate 
spectrographs  having  the  same  dispersion,  then  it  should  be 
easy  to  decide  which  method  will  furnish  heights  with  the 
maximum  of  accuracy  and  which  will  be  most  efficient  in  re- 
cording the  greatest  number  of  lines.  Of  course,  it  goes 
without  saying  that  the  dispersion  of  both  fixed  and  moving 
plate  spectrographs  should  be  as  great  as  possible.  A  com- 
parison is  not  now  being  made  of  the  relative  merits  of  prism 
or  grating  spectrographs  for  the  reason  that  either  type  of 
instrument  may  be  combined  with  either  fixed  or  moving 
plate. 

Under  ideal  conditions  of  perfect  timing  and  circular  edges 
of  both  moon  and  sun  (the  moon  without  mountains  and  val- 
leys, the  sun  without  prominences),  the  flash  spectrum  ap- 
pears at  the  instant  that  the  whole  photosphere  is  covered  up 
by  the  moon,  hence  with  edges  of  sun  and  moon  tangent  as 
in  Figure  6.  The  fixed  plate  records  the  crescent  arcs.  A 
knowledge  of  the  angular  diameter  of  the  sun  and  the  aug- 
mented diameter  of  the  moon  furnishes  a  ready  method  of 
deriving  from  the  spectra  the  heights  to  which  the  chromo- 
sphere vapors  extend.  Measurements  are  made  of  the  angu- 
lar length  of  the  different  arcs,  or  more  simply  by  the  meas- 
urement of  the  half -chord  of  this  arc  yo  in  Figure  6.  Under 
the  ideal  conditions  outlined  above,  the  chromospheric  arcs  at 
their  middle,  or  at  the  horizontal  line  in  Figure  6,  represent 
in  each  spectral  line  the  radiations  from  all  levels  of  the 


PHOTOGRAPHING    THE    FLASH    SPECTRUM     263 

chromosphere  lying  above  the  photosphere.  At  a  height  of 
y<>  equal  to  2.0  mm  on  the  original  spectra  of  1905  (with  a 
focal  length  of  60  inches)  each  of  the  crescent  arcs  repre- 
sents the  chromosphere  at  levels  that  are  above  700  kilo- 
meters, a  half-chord  of  3.0  mm  in  each  case  gives  the  condi- 
tions at  levels  above  1600  km.  At  a  value  of  y<>  equal  to  5.0 


FIG.  6    Measuring  the  heights  of  solar  vapors. 

mm,  the  tip  of  a  high  level  cusp  is  reached  and  this  tip  fur- 
nishes a  detectable  amount  of  radiation  impressed  on  the 
photographic  plate.  At  the  tip  of  this  chromospheric  arc, 
therefore,  only  those  radiations  reach  the  photographic  plate 
which  come  from  emitting  atoms  above  the  5000  km  level. 
Hence  each  chromospheric  arc,  both  above  and  below  the 
middle  of  the  arcs  as  recorded  in  the  spectrum,  gives  a  record 
of  the  radiations  at  the  photosphere  and  at  higher  and  still 
higher  levels  as  the  tips  of  the  arcs  are  approached.  If  the 
tip  of  the  chromospheric  arc  comes  at  a  half-chord  measured 
at  2.0  mm,  then  the  radiations  above  the  700  km  level  are 
sufficiently  active  to  give  a  blackening  of  the  photographic 
image  strong  enough  to  be  detected.  The  "  height  "  is  then 
said  to  be  700  kilometers.  Under  the  actual  conditions  at 


264  ECLIPSES    OF    THE    SUN 

the  time  of  the  eclipse,  neither  the  edge  of  the  moon  nor  that 
of  the  sun  is  circular  in  outline,  nor  is  the  exposure  of  the 
flash  spectrum  an  instantaneous  one,  and  the  scale  of  heights 
is  accordingly  modified.  The  moon  has  mountains  and  val- 
leys projected  against  the  edge  of  the  sun,  which  has  promi- 
nences. The  chromospheric  arcs  will  present  a  picture  of 
these  effects.  If  these  arcs  are  very  irregular,  then  mani- 
festly it  would  be  difficult  to  get  reliable  information  re- 
garding heights,  particularly  if  a  large  and  active  prominence 
is  projected  near  the  tips  of  the  chromospheric  arcs. 

With  the  moving  plate  method,  all  features  of  the  chromo- 
spheric arc  are  exactly  as  for  the  fixed  plate  outline  above, 
the  only  difference  being  that  the  slot  used  with  moving  plate 
concentrates  attention  on  a  definite  section  of  the  chromo- 
spheric arc.  With  the  arrangement  used  at  the  1905  eclipse, 
the  slot  width  uncovered  a  layer  of  300  km  at  one  time.  As 
the  eclipse  progressed  the  moving  plate  furnished  an  inte- 
grated picture  of  what  was  happening  within  this  layer  300 
km  in  height. 

In  the  earlier  editions  of  this  book,  and  also  in  the  memoir 
in  Volume  IV  of  Handbuch  der  Astrophysik,  the  author  has 
stated  that  one  decided  objection  to  the  moving  plate  method 
is  that  at  the  time  of  an  eclipse  the  slot  might  readily  be 
superposed  on  a  section  of  the  sun  where  there  was  an  erup- 
tive prominence.  At  the  eclipse  of  1918,  the  "  Heliosaurus  " 
shown  facing  page  196  was  exactly  at  the  position  of  third 
contact.  At  the  eclipse  of  1919,  another  large  prominence 
group  is  shown  facing  page  272. 

The  Lick  observers  recognize  the  force  of  this  criticism  as 
is  shown  by  the  following  (loc.  cit.  p.  249),  "  One  valid  ob- 
jection to  the  moving  plate  is  that,  if  the  image  of  a  promi- 
nence falls  on  the  plate,  the  resultant  heights  would  be  diffi- 
cult to  interpret."  Hence  at  the  latest  eclipse  observed  with 
moving  plate,  that  of  April  28,  1930,  the  Lick  observers  ex- 
amined the  solar  image  just  before  the  beginning  of  totality 
in  an  effort  to  set  the  slot  on  a  portion  of  the  sun  devoid  of 
large  disturbances.  One  unfortunate  feature  resulting  from 
this  attempt  *  was  the  failure  to  photograph  the  layers  of 

1  Publ.  A.  S.  P.,  42,  131,  1930. 


PHOTOGRAPHING    THE    FLASH    SPECTRUM     265 

lowest  level  owing  to  the  plate  not  being  put  in  motion  until 
after  the  total  phase  was  well  started. 

Regarding  the  relative  number  of  lines  photographed  by 
fixed  or  moving  plates,  it  seems  easy  to  reach  conclusions.  In 
the  region  of  wave-lengths,  from  4200  to  4500  A,  where  all 
three  spectra  had  the  best  definition,  the  1905  and  1908  mov- 
ing plates  recorded  425  and  292  lines  respectively,  while  in 
the  same  region  of  wave-lengths,  the  1905  concave  grating 
spectra  had  43  7  lines ;  and  this  in  spite  of  the  fact  that  the 
moving  plates  had  average  dispersions  twice  and  four  times, 
respectively,  that  of  this  1905  fixed  plate.  Between  the  re- 
gion 3900  and  4100  A,  the  Lick  1905  moving  plate  recorded 
413  lines  and  the  Lick  1905  fixed  plate  454  lines  in  spite  of 
the  better  definition  of  the  former  and  a  dispersion  more 
than  twice  greater  than  that  of  the  latter.  Hence  with  equal 
dispersion  and  definition  the  fixed  plate  will  always  show 
more  lines  than  a  moving  plate. 

Another  disadvantage  of  the  moving  plate  is  the  difficulty 
of  ascertaining  what  zero  to  adopt  as  the  initial  level  for  the 
scale  of  heights.  This  zero-point  depends  on  the  reversal 
from  dark  to  bright  lines  on  the  photograph  (at  first  flash) 
or  the  reverse  at  second  flash.  As  already  stated,  at  the  be- 
ginning of  totality,  the  high-level  lines  become  bright  many 
seconds  before  those  of  lower  levels.  The  reversal  of  the 
lines  of  lowest  level  forms  the  "  flash  "  spectrum.  With 
the  fixed  plate,  measures  of  the  lengths  of  the  arcs  give  the 
heights  directly,  but  with  the  moving  plate,  the  photographs 
themselves  must  furnish  the  zero-point  from  the  reversal  of 
the  lines.  This  is  a  difficult  task  which  is  further  compli- 
cated by  the  fact  that  the  slot  causes  an  effect  integrated 
over  a  region  stretching  over  300  km  in  height.  It  would 
seem  therefore  that  this  integrated  effect  would  not  permit  a 
high  precision  in  the  determination  of  heights  by  the  mov- 
ing plate  which  must  also  be  subject  to  a  zero-point  correc- 
tion far  larger  than  with  the  fixed  plate. 

In  making  comparisons  of  the  reliability  of  fixed  and  mov- 
ing plates,  the  best  conclusion  can  be  reached  by  confining 
attention  to  the  low-level  lines  only,  those  that  reach  heights 
of  700  km  or  less,  these  lines  being  vastly  greater  in  number 


266  ECLIPSES    OF    THE    SUN 

than  those  of  higher  levels  and  their  heights  are  known  with 
greater  precision.  The  Lick  1905  fixed  plate  gave  average 
heights  for  these  lines  of  lowest  levels  which  differed  about 
50  km  from  those  of  the  1905  Mitchell  plate.  On  the  other 
hand,  the  average  heights  from  these  same  low-level  lines 
from  the  1905  moving  plate  were  400  km  greater  than  those 
from  the  Lick  1905  fixed  plate.  At  one  and  the  same  eclipse, 
the  simplest  method  of  interpreting  this  systematic  differ- 
ence is  the  zero-point  correction  of  the  moving  plate.  More- 
over, the  1905  moving  plate  gave  average  heights  for  the  low 
levels  more  than  500  km  greater  than  the  1908  moving 
plate. 

Furthermore,  with  the  moving  plate,  the  scale  of  heights 
depends  on  an  accurate  knowledge  of  the  speed  of  motion 
of  the  plate  and  also  on  the  assumption  that  this  motion  is 
absolutely  uniform. 

Considering  therefore  the  difficulties  outlined  above  and 
the  various  causes  of  errors  that  may  beset  the  moving  plate 
method,  the  author  is  in  hearty  agreement  with  the  opinion 
expressed  (loc.  cit.,  p.  2)  that  "  it  is  doubtful  if  any  other 
astrophysical  instrument  is  as  difficult  to  set  up  and  operate 
successfully  as  an  objective-prism  eclipse  spectrograph,  when 
used  with  a  continuously  moving  photographic  plate  includ- 
ing a  slit  immediately  in  front  of  the  plate."  One  might  also 
add  that  it  is  doubtful  if  the  photographs  of  any  other  astro- 
physical  instrument  are  so  difficult  to  interpret  in  order  to 
reach  conclusion  free  from  systematic  errors. 

If  the  flash  spectrum  were  an  exact  reversal  of  the  Fraun- 
hofer  lines,  both  as  regards  wave-lengths  and  intensities, 
the  eclipse  observations  could  add  but  little  to  our  knowl- 
edge of  solar  physics.  In  such  a  case,  the  precious  moments 
of  a  total  eclipse  should  be  devoted  to  the  investigation  of 
other  lines  of  research.  In  fact,  the  flash  spectrum  is  inter- 
esting and  important  only  in  so  far  as  it  differs  from  spectra 
taken  under  ordinary  conditions. 

The  intensity  of  a  spectrum  line  depends  both  on  the  width 
and  on  the  blackness  of  the  photographic  image  of  the  line. 
It  is  unfortunate  that  in  all  spectra,  whether  of  dark  or  bright 
lines,  whether  determined  in  the  laboratory  or  in  the  observa- 


PHOTOGRAPHING    THE    FLASH    SPECTRUM     267 

tory,  it  is  ordinarily  impossible  to  have  a  scale  for  the  desig- 
nation of  relative  intensities  which  is  other  than  arbitrary. 
With  such  a  scale  the  strongest  line  in  any  spectrum  may  be 
represented  by  10,  by  100,  or  even  by  1000,  while  the  weak- 
est line  receives  the  number  i,  or  o, —  i, —  2,  or  even 
—  3,  as  in  the  revised  Rowland's  Tables.  Such  scales  being 
arbitrary  may  not  be  uniform,  and  it  is  consequently  very 
difficult  to  compare  the  values  of  the  intensities  of  one  spec- 
trum assigned  by  one  observer  with  those  investigated  by 
another. 

For  reasons  outlined  above,  with  chromospheric  arcs  pho- 
tographed by  the  fixed  plate,  it  is  readily  possible  to  make 
estimates  of  intensities  or,  better  still,  measures  of  intensities 
by  the  microphotometer  at  different  positions  along  the  arcs. 
At  the  middle  of  the  arcs  the  intensities  refer  to  the  radiations 
of  all  atoms  above  the  photosphere.  At  positions  2  mm  from 
the  middles  of  the  arcs  the  intensities  are  thus  derived  from 
the  radiation  from  all  atoms  above  700  km  in  height.  By 
estimating  or  measuring  the  intensities  at  different  places 
along  the  chromospheric  arcs,  from  their  middle  to  the  tips  of 
the  cusps,  a  knowledge  is  obtained  of  the  radiations  of  atoms 
at  higher  and  still  higher  levels  above  the  photosphere.  In 
similar  manner  with  the  moving  plate,  estimates  of  inten- 
sities may  be  made  or  measures  with  the  microphotometer 
be  carried  out,  as  was  done  by  Menzel  (loc.  cit.}.  With  the 
moving  plate,  as  with  the  fixed  plate,  the  intensities,  esti- 
mated or  measured,  refer  always  to  the  radiations  from  atoms 
above  certain  levels  as  determined  by  the  edge  of  the  ad- 
vancing moon. 

As  has  been  stated,  with  the  fixed  plate,  the  interpreta- 
tion of  heights  is  complicated  by  the  Baily's  beads  which 
make  knots  in  the  curved  lines  of  the  spectrum.  However,  in 
estimating  or  measuring  intensities  on  the  fixed  plate  at  dif- 
ferent heights  above  the  zero-point,  the  observer  has  it  quite 
in  his  power  to  make  measures  at  any  particular  location 
along  the  whole  of  the  curved  arc.  Hence  the  Baily  beads 
need  be  of  no  particular  concern  unless  the  solar  disturb- 
ance is  great  in  size  and  extent.  With  the  moving  plate,  on 
the  contrary,  the  slot  has  a  fixed  definite  position  and  meas- 


268  ECLIPSES    OF    THE    SUN 

ures  may  be  made  of  conditions  affecting  the  chromosphere 
at  this  and  at  no  other  location. 

The  most  characteristic  difference  between  the  chromo- 
spheric  and  the  Fraunhofer  spectra  is  found  in  the  relative 
intensities  of  the  lines.  The  system  of  intensities  adopted 
for  the  chromospheric  spectrum  is  purely  an  arbitrary  one, 
in  which  200  represents  the  strongest  lines  like  K  and  Hy, 
and  o  that  of  the  weakest  line.  Naturally  the  intensities 
depend  on  the  character  of  the  photographic  plate  used,  but 
partial  allowance  may  be  made  for  the  decrease  in  sensitive- 
ness of  the  plate  in  the  green  and  yellow  regions.  In  estimat- 
ing intensities,  one  is  unconsciously  influenced  by  the  breadth 
of  the  lines,  so  that  the  values  for  intensity  give  a  somewhat 
combined  appraisal  of  the  blackness  and  breadth  of  a  cer- 
tain line.  These  at  best  are  but  estimates,  but  they  are 
perhaps  comparable  in  accuracy  with  estimates  of  intensities 
by  others,  for  instance,  as  in  the  Rowland's  Tables.  Esti- 
mates in  Rowland  and  in  the  flash  spectrum  may  1  be  cali- 
brated. 

The  reasons  for  these  pronounced  differences  in  intensi- 
ties between  the  dark  line  Fraunhofer  spectrum  and  the 
chromospheric  spectrum  will  be  evident  on  a  moment's  re- 
flection. Consider  a  Fraunhofer  line  coming  from  the  center 
of  the  sun's  surface,  and  assume  that  the  absorption  is  caused 
by  a  reversing  layer  500  miles  in  thickness.  The  light  com- 
ing perpendicularly  from  the  photosphere  can  be  absorbed 
by  the  atoms  in  this  5oo-mile  layer.  At  the  time  of  an  eclipse, 
the  light  of  the  chromosphere  comes  tangentially  from  the 
sun's  surface,  and  not  perpendicularly,  with  the  result  that 
the  chromospheric  light  is  affected  by  a  depth  of  20,000  miles 
of  atoms.  (The  line  of  sight  from  a  layer  500  miles  in  thick- 
ness passes  through  20,000  miles  of  solar  atmosphere  when 
tangent  to  the  sun's  surface.)  This  has  an  important  bearing 
on  Saha's  theory,  as  will  be  explained  in  Chapter  XVII. 

Researches  on  the  structure  of  the  atom,  as  shown  in 
Chapter  XIV  have  demonstrated  the  important  fact  that  the 
intensity  of  a  line  in  any  spectrum  depends  primarily  on  the 
number  of  emitting  or  absorbing  atoms.  In  the  Fraunhofer 

1  Astro  physical  Journal,  68,  i,  1928. 


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THE  CITADEL  CROWNING  THE  CITY  OF  DAROCA 


OFFICERS  AND  SAILORS  OF  THE  U.  S.  NAVY  ASSIST  THE  ASTRONOMERS  IN 
OBSERVING  THE  TOTAL  ECLIPSE  IN  SPAIN 


PHOTOGRAPHING    THE    FLASH    SPECTRUM    269 

spectrum  the  atoms  are  absorbing  radiation  but  in  the 
chromospheric  spectrum  they  are  emitting  radiation.  Let  us 
now  consider  two  different  elements  in  the  sun's  envelope; 
one  of  these  elements  has  a  low  density  but  extends  high  in 
miles  above  the  photosphere;  the  other  element  is  much 
heavier  and  its  atoms  are  confined  to  layers  lying  much  closer 
to  the  sun's  surface.  It  is  easy  to  imagine  in  the  case  of  ab- 
sorption of  radiation  that  the  total  number  of  atoms  involved 
in  producing  a  line  in  the  spectrum  of  the  lighter  vapor  might 
be  identical  to  the  number  contributing  to  the  dark  line  in 
the  heavier  vapor  on  the  condition  where  the  light  passes 
radially  through  the  gases,  for  instance,  when  coming  from 
the  center  of  the  sun's  disk.  If ,  however,  at  the  time  of  a  total 
eclipse  when  the  moon  covers  the  photosphere,  the  atoms  of 
the  chromosphere  emitting  radiation  come  tangentially  from 
the  sun's  envelope.  Under  these  circumstances,  the  greater 
height  attained  by  the  atoms  by  the  rarer  of  the  two  vapors 
adds  enormously  to  the  number  of  atoms  encountered  rela- 
tive to  the  number  encountered  in  the  low-lying  and  denser 
layer  when  the  atoms  are  excited  and  radiate  light.  Hence,  it 
is  readily  seen  that  although  the  two  gases  may  give  lines  of 
equal  intensity  in  their  absorption  spectra,  they  will  not 
necessarily  do  so  in  their  emission  spectra;  the  low-lying 
heavy  vapor  will  give  in  the  chromospheric  spectrum  short 
arcs,  while  the  other  assumed  vapor  will  give  longer  arcs  of 
greater  relative  intensity.  Though  there  are  many  contribut- 
ing causes,  the  main  reason  for  the  great  differences  in  in- 
tensities between  the  dark  and  bright  line  spectrum  of  the 
sun  is  the  differences  in  heights  to  which  the  vapors  extend. 
The  H  and  K  lines  of  enhanced  calcium  and  the  lines  of  the 
hydrogen  series  are  the  strongest  lines  in  the  chromosphere 
mainly  for  the  reason  that  the  atoms  of  enhanced  calcium  and 
hydrogen  are  detected  at  greater  elevations  than  are  attained 
with  any  of  the  other  elements. 

As  a  matter  of  fact,  there  are  such  enormous  differences 
in  the  intensities  of  the  Fraunhofer  and  flash  spectra  that 
placed  side  by  side,  as  they  are  on  page  268,  the  spectra 
seem  to  belong  to  stars  of  two  different  types  rather  than 
to  the  same  object  under  different  conditions.  It  is  these 


270  ECLIPSES    OF    THE    SUN 

differences  that  make  observations  of  eclipse  spectra  of  the 
greatest  value  in  widening  our  knowledge  of  solar  physics. 
The  chief  differences  in  intensity  for  the  stronger  lines  are 
found  in  the  elements  helium  and  hydrogen.  As  is  well 
known,  no  helium  absorption  lines  are  ordinarily  found  in 
the  sun,  whereas  in  the  eclipse  spectrum  the  helium  lines 
are  conspicuous  by  their  great  strength.  In  the  Fraunhofer 
spectrum  there  are  only  four  hydrogen  lines  visible,  while  in 
the  flash  spectrum  there  is  the  whole  Balmer  series,  no  less 
than  thirty- four  lines  being  measured  on  the  1905  plates. 

One  can  therefore  see  at  a  glance  the  very  great  importance 
attaching  to  a  knowledge  of  heights  in  the  chromosphere. 
The  following  chapters  will  show  how  this  information  not 
only  has  been  the  basis  of  Saha's  Theory  of  lonization  but 
in  addition  has  had  an  important  practical  bearing  on  many 
solar  and  stellar  problems. 

This  close  comparison  of  the  solar  and  chromospheric  spec- 
tra side  by  side  helped  make  the  identifications  of  sources 
rather  certain.  But  the  Rowland  spectrum  had  a  dispersion 
fully  ten  times  the  dispersion  of  the  chromospheric  spectrum 
(2i-foot  radius  in  the  second  order  compared  with  5-foot 
focus  in  first  order,  the  gratings  having  nearly  the  same  num- 
ber of  lines  per  inch).  Naturally,  lines  which  appear  single 
in  the  chromospheric  spectrum  may  be  a  blend  of  two  or 
more  lines  with  the  greater  dispersion.  But  lines  which  ap- 
pear as  a  close  pair  or  a  blend  in  the  chromosphere  must  be 
the  result  of  the  blend  of  corresponding  lines  in  Rowland. 
On  account  of  the  great  differences  in  intensity  of  the  chro- 
mospheric and  Rowland  spectra,  it  was  difficult  to  be  always 
sure  of  identifications  until  photographs  were  compared  side 
by  side.  The  original  photograph  of  the  flash  spectrum  was 
enlarged  five  times.  Rowland's  great  Atlas  was  reduced  six 
times.  Since  the  flash  spectrum  was  nearly  normal,  it  was 
possible  to  procure  both  spectra  on  a  comparatively  close 
approximation  to  scale.  This  comparison  of  spectra  will 
perhaps  speak  more  strongly,  than  any  words  or  comparison 
of  wave-lengths,  concerning  the  sharpness  of  the  original 
spectrum  of  the  chromosphere.  On  account  of  the  small 
variations  from  the  normal  spectrum  (noted  above)  it  was 


PHOTOGRAPHING    THE    FLASH    SPECTRUM    271 

impossible  to  obtain  an  exact  match  in  scale  in  the  two  pho- 
tographs. Those  who  are  interested  sufficiently  will  be  able 
to  carry  the  comparison  along  line  for  line. 

Repeated  attention  must  be  directed  to  the  fact  that  the 
heights  of  the  chromospheric  vapors  determined  from  the 
measurement  of  the  angular  length  of  the  cusps,  can  afford 
no  great  accuracy  in  the  determination  of  the  absolute  heights 
in  kilometers  to  which  the  various  layers  extend  above  the 
photosphere.  The  method  depends  on  the  visibility  of  the 
ends  of  the  cusps.  It  is  quite  possible,  and  probable,  that 
vapors  extend  in  detectable  amounts  to  elevations  beyond 
the  limits  visible  in  the  cusps.  The  heights  derived  by  this 
method  can  therefore  only  represent  a  mean  height  and  can- 
not be  expected  to  furnish  the  maximum  heights  to  which 
the  vapors  in  detectable  amounts  extend.  Attention  should 
likewise  be  called  to  the  fact  that  the  heights  measured  in 
this  manner  cannot  give  the  elevations  above  the  photo- 
sphere, but  rather  above  the  average  level  of  the  layer  pho- 
tographed in  this  particular  flash  spectrum.  With  these  limi- 
tations, the  method  is  capable  of  furnishing  the  relative 
heights  of  the  layers  producing  the  individual  spectrum  lines 
with  a  fair  degree  of  accuracy. 

This  is  not  the  place  to  go  into  greater  technical  details 
which  may  be  found  in  Publications  of  the  Lcander  McCor- 
mick  Observatory,  Vol.  5,  part  7,  1932,  or  in  the  Astrophysi- 
cal  Journal.  However,  a  brief  summary  may  be  made  here 
of  the  advantages  and  disadvantages  of  the  moving  over  the 
fixed  plate  for  the  spectrum  of  the  chromosphere.  It  is  as- 
sumed that  the  two  types  of  spectrographs  have  about  equal 
dispersion  and  light  gathering  power.  Wave-lengths  are  ob- 
tained with  equal  accuracy  by  the  two  methods  and  hence 
may  be  left  out  of  consideration. 

The  moving  plate  is  a  much  more  complicated  instrument 
and  demands  greater  technical  skill  in  the  erection  and  ad- 
justment at  the  eclipse  site.  In  photographing  the  flash 
spectrum,  the  moving  plate  has  distinct  advantages  in  the 
proper  timing  of  the  exposures.  The  moving  plate  shows  the 
gradual  change  from  dark  to  bright  lines  at  the  first  flash  (or 
the  reverse  on  second  flash).  The  separate  exposures  with 


272  ECLIPSES    OF    THE    SUN 

the  fixed  plate  make  a  similar  but  less  complete  record.  With 
equal  dispersion  and  definition,  the  fixed  plate  records  more 
lines  than  the  moving  plate. 

If  the  chromosphere  is  greatly  disturbed  near  the  posi- 
tions of  second  and  third  contacts,  the  heights  recorded  from 
the  lengths  of  the  arcs  on  the  fixed  plates  are  difficult  to  inter- 
pret, especially  if  a  solar  prominence  occurs  near  one  of  the 
cusps.  With  the  moving  plate,  the  slot  which  defines  the 
region  of  investigation  may  be  superposed  on  an  active  promi- 
nence region  and  therefore  the  photographs  will  not  repre- 
sent average  solar  conditions.  For  the  first  flash,  the  slot 
might  possibly  be  set  immediately  before  totality  so  as  to 
avoid  disturbed  regions  in  the  chromosphere.  This  however 
could  not  be  done  for  second  flash.  The  present  writer  would 
never  under  any  circumstances  attempt  such  an  operation. 
It  is  far  too  difficult  a  task  to  undertake  in  the  few  excited 
seconds  available.  It  is  far  easier  to  observe  the  appearance 
of  the  flash  spectrum  with  the  binoculars  and  direct-vision 
spectroscope,  and  then  to  start  and  close  the  exposures  ac- 
cordingly. 

If  successful  photographs  are  obtained  with  both  fixed  and 
moving  plates,  it  is  much  easier  to  interpret  the  results  of  the 
former.  With  the  arcs  recorded  by  the  fixed  plate,  any  por- 
tion of  them  may  be  adopted  for  discussion.  The  levels  that 
lie  closest  to  the  photosphere  manifest  themselves  by  the 
appearance  of  a  strong  continuous  spectrum  on  the  photo- 
graph. Hence  with  the  fixed  plate,  the  zero-point  correction 
should  be  small,  probably  of  the  order  of  50  kilometers.  At 
the  1905  eclipse,  the  Lick  fixed  plate  at  second  contact  and 
the  Mitchell  fixed  plate  at  third  contact  gave  low-lying  levels 
which  differed  about  50  km,  those  of  the  latter  recording  the 
greater  heights. 

With  the  moving  plate,  the  zero-point  correction  depends 
on  the  estimation  of  the  location  where  on  the  photograph  the 
spectral  lines  reverse  from  dark  to  bright  (or  bright  to  dark). 
The  zero-level  should  be  that  of  the  vapors  closest  to  the 
photosphere,  those  of  lowest  level.  If  there  are  many  en- 
hanced lines  in  the  spectrum,  as  in  the  region  of  4300  and 
4400  A,  the  problem  becomes  complicated  and  more  diffi- 


THE   GREAT  PROMINENCE  OF  MAY   29,   1919 
Photographed  at  Yerkes  Observatory  by  Pettit  with  the  spectroheliograph 

nn  tnr>  Ha\r  nf  fl-i«  f/^tol  «^K»^o«.       T>U» -L__.L  i_^-    i  ,       ,.     .        .  .  ^e>*c*i-'*i 


on  the  day  of  the  total  eclipse. 


The  greatest  height  attained  on  that  day  was 
760,000  kms. 


BRITISH  PHOTOGRAPH  OF  TOTAL  ECLIPSE  OF  MAY  29,  1919 

The  same  prominence  is  shown  facing  page  272  from  photographs  taken  at  Yerkes 
Observatory  where  there  was  no  eclipse, 


PHOTOGRAPHING    THE    FLASH    SPECTRUM    273 

cult.  As  already  stated,  the  Lick  1905  moving  plate  gave 
the  heights  of  the  vapors  of  lowest  level  400  km  in  excess  of 
the  Lick  1905  fixed  plate.  This  systematic  difference  indi- 
cates zero-point  corrections  which  are  larger  for  moving  than 
for  fixed  plates. 

The  1905  Lick  moving  plate  had  a  slot  width  of  .05  inches 
which  permitted  radiation  from  300  kms  of  vapors  to  pass 
through  and  be  recorded  on  the  photograph.  The  motion  of 
the  plate  causes  progressive  exposures  to  be  made  along  the 
spectral  line.  The  height  of  the  vapor  is  derived  in  each  case 
from  the  total  length  of  the  line  measured  from  the  assumed 
zero-point  up  to  the  vanishing  tip.  But  this  total  length 
depends  on  the  integrated  effect  of  the  radiation  passing 
through  the  slot  of  the  mechanism.  Hence  it  would  seem 
that  the  moving  plate  might  readily  have  systematic  differ- 
ences between  the  low-level  heights  and  those  of  greater  ele- 
vation, and  this  is  further  complicated  by  departures  of  the 
motion  of  the  plate  from  a  uniform  character.  "  An  examina- 
tion of  the  1905  eclipse  plate  indicates  that  in  the  neighbor- 
hood of  1800  kms,  the  plate  had  experienced  a  slight  accelera- 
tion "  (loc.  cit.,  p.  258). 

In  regard  to  the  photometric  measures,  a  quotation  may 
be  made  (in  its  corrected  form)  by  Campbell  (loc.  cit.,  p. 
iv).  "  My  moving-plate  method  gives  the  average  or  in- 
tegrated effect  of  a  relatively  long  segment  of  the  chromo- 
spheric  crescent  itself,  say  300  km  long,  the  inner  edge  of 
which  is  serrated  by  lower  mountain  peaks.  The  fixed  plate 
method  records  the  serrated  crescent  as  a  whole  (Mitchell), 
or  in  part  (Pannekoek  and  Minnaert) ;  and  it  is  interesting 
to  note,  a  long  segment  of  the  serrated  photographic  crescent 
was  averaged  or  integrated  upon  a  moving  plate  of  the  re- 
cording microphotometer  by  Pannekoek  and  Minnaert  as 
the  basis  of  their  photometry  of  the  chromospheric  spectrum. 
In  their  photometric  applications,  therefore,  the  two  meth- 
ods seem  to  have  an  element  of  equivalence."  However,  we 
might  remind  Dr.  Campbell  that  in  the  measurement  of  the 
fixed  plate  spectra,  it  is  not  necessary  to  use  a  long  slit  with 
the  microphotometer.  A  short  slit  may  be  used  in  order  to 
integrate  the  measures  over  a  relatively  short  range  in  heights. 


274  ECLIPSES    OF    THE    SUN 

In  the  actual  operation  of  measuring  the  intensities  of  the 
spectra  of  fixed  or  moving  plates  there  should  be  practically 
no  difference  in  procedure. 

In  the  foregoing,  there  has  been  stressed  some  few  of  the 
difficulties  besetting  the  eclipse  observer.  No  matter  how 
carefully  and  precisely  the  focus  of  the  spectrograph  is  de- 
termined, no  matter  how  accurately  the  exposures  are  timed, 
either  by  fixed  or  moving  plate,  the  photographed  spectra 
may  not  be  of  finest  quality  for  causes  entirely  outside  the 
control  of  the  observer,  namely,  the  "  seeing."  As  an  ex- 
ample, the  experience  of  the  writer  may  be  cited.  At  the 
1925  eclipse  the  flash  spectrum  was  photographed  with  the 
same  equipment  as  at  the  1905  eclipse.  At  the  1925  eclipse, 
the  sun  was  only  17°  above  the  horizon,  and  in  consequence 
the  seeing  was  poor.  The  1905  eclipse  was  observed  under 
excellent  conditions  of  seeing.  The  1905  spectra  show  the 
faintest  lines  of  the  spectrum  in  much  greater  numbers  than 
the  1925  photographs,  the  lines  in  the  former  eclipse  being 
stronger  and  more  clearly  defined.  The  explanations  for  the 
differences  in  the  two  spectra  cannot  be  found  in  differences 
of  focus,  or  of  exposures,  or  of  levels  photographed.  Under 
the  poor  conditions  of  seeing  of  the  1925  eclipse,  the  light  of 
the  bright-lined  chromospheric  arcs  was  rendered  ill-defined 
by  being  spread  over  a  larger  area.  With  the  strong  lines  of 
the  spectrum,  this  effect  made  the  edges  hazy  but  diminished 
the  total  intensities  of  the  lines  but  little.  With  the  weak 
lines,  however,  the  spreading  of  the  light  over  a  larger  area  on 
the  plate  caused  a  marked  diminution  in  intensity  and  sharp- 
ness of  each  line  with  the  necessary  result  that  the  weakest 
lines  in  the  1925  spectrum  were  practically  obliterated  by 
the  poor  seeing  and  only  those  lines  of  a  certain  minimum 
strength  survived.  Astronomers  who  are  engaged  in  the  ob- 
servational work  of  stellar  photography,  particularly  with 
telescopes  of  great  focal  length,  are  entirely  familiar  with  an 
exactly  analogous  problem.  Under  the  best  conditions  of 
seeing,  the  star  images  are  hard  and  sharp  and  with  well- 
defined  edges.  Under  poorer  and  poorer  conditions  of  see- 
ing, the  star  images  increase  in  size  and  the  edges  become 
more  and  more  fuzzy.  The  larger  stellar  images  with  poor 


PHOTOGRAPHING    THE    FLASH    SPECTRUM    275 

seeing  require  an  increase  of  exposure  to  photograph  a  star 
of  any  given  magnitude.  The  length  of  exposure  becomes 
greater  and  greater  as  the  seeing  deteriorates  more  and  more. 
Under  poor  conditions,  the  star  images  on  the  plates  are 
large  and  ill-defined  and  the  accuracy  of  measurement,  when 
compared  with  plates  taken  under  good  conditions,  is  much 
diminished. 

If  photographs  of  the  flash  spectrum  are  actually  secured 
in  spite  of  all  difficulties  to  be  overcome,  then  comes  the 
much  more  important  problem  of  the  discussion.  The  size 
of  this  task  may  be  surmised  from  the  fact  that  Dr.  Menzel 
spent  about  five  years  in  preparing  for  publication  the  hand- 
some Lick  Publication,  Volume  17.  On  page  2,  it  took  one- 
half  a  quarto  page  for  him  to  render  thanks  to  those  individu- 
als or  bodies  who  had  given  scientific  or  financial  assistance. 
In  similar  manner  it  has  also  taken  the  present  writer,  al- 
though working  almost  alone  and  with  his  time  interrupted 
by  many  duties,  five  years  to  prepare  the  original  contribu- 
tion in  Publications  of  the  Leander  McCormick  Observa- 
tory, Volume  II,  parts  2  and  3  and  the  revision  in  Volume  V, 
parts  2,  3,  6  and  7. 

It  will  not  be  necessary  here  to  explain  in  further  detail 
the  methods  followed  in  deriving  wave-lengths  and  heights. 
This  information  may  be  obtained  from  the  sources  just 
cited. 

When  the  spectra  of  the  1905  eclipse  were  being  discussed 
by  the  writer,  comparatively  little  was  known  of  the  differ- 
ences between  the  conditions  existing  in  the  Fraunhofer 
spectrum  and  in  the  chromosphere.  The  main  difference 
known  to  exist  was  that  pointed  out  by  Fowler  from  the  1898 
eclipse  spectra,  namely,  that  the  enhanced  lines  played  an 
important  role  in  the  flash  spectrum  by  showing  increased 
intensities  over  those  in  the  solar  spectrum.  But  how  was 
one  to  find  the  wave-lengths  of  the  lines  which  had  greater 
strengths  in  the  spark  than  in  the  arc  spectrum?  Lockyer 
had  given  lists  of  some  of  these  lines,  and  these  were  good  as 
far  as  they  went.  But  the  known  lines  were  few  in  number 
and  entirely  inadequate  for  a  discussion  of  the  problem. 
What  was  to  be  done?  Kayser's  important  lists  had  not  yet 


276  ECLIPSES    OF    THE    SUN 

appeared.  There  was  nothing  to  do  but  to  follow  the  ex- 
ample set  by  Kayser  and  others,  namely,  that  of  going  to  the 
original  sources  one  by  one.  There  seemed  no  other  method, 
and  this  meant  many  long  and  tedious  hours  of  patient  com- 
parisons. 

Within  the  past  decade,  however,  the  combined  attack  on 
the  structure  of  the  atom  by  the  astronomer,  physicist  and 
chemist  has  resulted  in  a  stupendous  increase  in  knowledge 
of  conditions  underlying  the  production  of  emission  and  ab- 
sorption lines  in  different  spectra.  The  outstanding  achieve- 
ments have  been  the  Bohr-Sommerfeld  atom,  Saha's  theory 
of  ionization,  investigations  of  series  in  spectra  coming  mainly 
from  the  hands  of  Fowler,  Russell  and  Meggers,  and  the 
publication  from  Mount  Wilson  Observatory  of  the  Revision 
of  Rowland's  Table  of  Solar  Wave-Lengths.  As  a  result  of 
this  greatly  increased  information  now  available,  particularly 
in  the  proper  identifications  of  the  sources  causing  the  spec- 
tral lines,  the  discussion  of  the  flash  spectrum  has  become 
one  of  greatly  increased  simplicity. 


CHAPTER  XVI 

DISCUSSING  THE  FLASH  SPECTRUM 

A"  |  AHE  depths  of  the  various  layers  of  gases  surround- 
I  ing  the  sun  have  an  importance  which  is  very  vital 
JL  in  all  theories  of  solar  physics.  Eclipse  spectra  fur- 
nish the  only  means  yet  known  of  directly  measuring  these 
depths.  Comparisons  of  the  flash  spectrum  with  the  solar 
spectrum  taken  under  ordinary  conditions  reveal  many  im- 
portant conclusions,  the  first  of  which  is  that  wave-lengths 
are  identical  in  the  flash  and  in  the  Fraunhofer  spectrum. 
This  statement  can  be  true  only  within  the  limits  of  accuracy 
attainable  in  the  measurements  of  wave-lengths  of  the  flash 
spectrum,  or  expressed  in  other  terms,  it  may  be  said  that  no 
differences  between  the  two  spectra  exceed  0.02  angstroms. 
The  flash  spectrum  must  therefore  be  regarded  as  a  true  re- 
versal of  the  ordinary  spectrum  of  the  sun  since  every  strong 
line  in  the  solar  spectrum,  without  any  exception,  is  changed 
to  a  bright  line  at  the  beginning  and  at  the  ending  of  totality. 
Although  the  wave-lengths  in  the  two  spectra  are  identi- 
cal, this  is  far  from  being  true  with  the  relative  intensities 
of  the  lines.  In  the  flash,  many  lines  appear  which  are  not 
found  in  the  ordinary  solar  spectrum;  furthermore,  some 
strong  lines  in  the  Fraunhofer  spectrum  appear  as  weaker 
lines  in  the  flash,  and  vice  versa,  weak  lines  in  the  sun  are 
strengthened  in  the  spectrum  of  the  chromosphere.  Dif- 
ferences in  intensity  signify  differences  in  level  and  in  elec- 
trical, thermal  and  pressure  conditions  of  the  vapors  pro- 
ducing the  lines,  and  consequently  an  intimate  study  of  such 
dissimilarities  will  give  valuable  information  regarding  the 
distribution  of  atoms  in  the  solar  atmosphere. 

The  problem,  however,  is  a  complicated  one  for  the  reason 
that  the  intensities  depend  so  intimately  on  the  level  of  the 
chromosphere  photographed,  all  spectral  lines  being  most 

277 


278  ECLIPSES    OF    THE    SUN 

intense  at  levels  closest  to  the  photosphere.  Hence,  at  one 
and  the  same  eclipse  and  with  exactly  the  same  timing,  dif- 
ferent intensities  are  found  on  the  photographs  depending 
on  whether  the  spectrograph  is  used  with  or  without  slit. 
Without  a  slit,  all  levels  contribute  their  emission  to  the 
spectral  line,  while  with  a  slit  only  those  levels  covered  by 
the  slit  give  up  their  light  to  the  line  in  question.  Mani- 
festly, the  position  of  the  image  of  the  slit  with  respect  to 
the  photosphere  is  of  prime  importance,  and  also  whether 
or  not  there  is  a  prominence  at  the  sun's  edge  where  the  slit 
is  projected.  Hence  with  a  slit,  there  are  many  reasons  why 
the  intensities  of  the  flash  lines  estimated  or  measured  from 
the  photographs  of  any  one  eclipse  may  vary  from  photo- 
graph to  photograph  or  from  instrument  to  instrument.  Even 
on  the  same  photograph  there  are  differences  of  intensities 
for  the  same  line.  In  their  publication  dealing  with  the  1926 
eclipse,  Davidson  and  Stratton  give  the  estimates  at  two  dif- 
ferent positions  in  their  photographs,  called  respectively 
"  flash  "  and  "  prominence."  From  their  photographs  taken 
at  the  1927  eclipse  with  slit,  Pannekoek  and  Minnaert  note 
similar  differences  in  intensities. 

For  the  same  reasons,  eclipse  spectra  taken  without  slit 
show  wide  ranges  in  intensities  depending  on  the  levels  pho- 
tographed. For  instance,  the  author  secured  spectra  with 
the  same  equipment  in  1905  and  1925.  The  intensities  dif- 
fer materially  in  the  two  years,  and  in  turn  they  both  differ 
in  most  details  from  the  intensities  observed  at  the  1926  and 
1927  eclipses,  and  also  from  the  Lick  results  at  the  eclipses 
of  1898,  1900,  1905  and  1908.  One  should,  therefore,  be 
very  guarded  when  comparing  one  eclipse  with  another  be- 
fore coming  to  the  conclusion  that  differences  in  intensities 
between  two  eclipses  mean  differences  in  solar  activity. 
Manifestly  one  should  not  push  too  far  any  deductions  based 
on  the  intensities  at  any  one  eclipse. 

Since  the  date  of  the  second  edition  of  this  book,  several 
important  discussions  have  appeared  dealing  with  the  ob- 
servations of  the  flash  spectrum.  The  instrumental  equip- 
ment has  been  described  in  the  preceding  chapter.  A  list  of 
the  more  extended  of  these  publications  follows,  arranged  in 


DISCUSSING    THE    FLASH    SPECTRUM  279 

order  of  the  dates  of  the  eclipses  observed.  As  already  stated, 
Campbell  with  the  moving  plate  obtained  photographs  in 
1898,  1900,  1905  and  1908.  In  1905,  with  the  fine  seeing  of 
the  Spanish  eclipse,  the  spectrum  also  was  photographed  on 
a  fixed  plate.  The  discussion  by  Menzel  makes  a  veritable 
storehouse  of  valuable  information.  Accurate  wave-lengths 
derived  from  the  weighted  mean  of  the  different  spectra  make 
possible  a  close  comparison  with  Rowland.  The  identifica- 
tion of  the  individual  lines  are  given  in  splendid  detail  to- 
gether with  their  multiplet  designation.  The  intensities  were 
estimated  on  the  Rowland  scale.  For  certain  of  the  stronger 
lines,  the  intensities  were  measured  by  the  microphotometer 
at  different  heights  above  the  photosphere.  On  the  observa- 
tional side,  the  most  important  information  is  contained  in 
the  heights  from  both  fixed  and  moving  plates.  As  shown 
in  the  preceding  chapter,  the  heights  from  the  moving  plates 
are  subject  to  larger  systematic  errors  than  those  from  fixed 
plates.  After  tabulating  the  material  in  order  of  wave- 
lengths, a  further  tabulation  rearranges  the  material  accord- 
ing to  the  source  of  the  line,  a  grouping  being  made  accord- 
ing to  the  multiplet  structure.  A  summary  of  the  various 
recent  theories  underlying  the  problems  of  solar  physics 
makes  this  Lick  publication  an  epoch-making  contribution. 
As  an  illustration  of  the  tremendous  activity  since  1923  (the 
date  of  the  first  edition  of  this  book)  caused  by  researches 
on  the  structure  of  the  atom,  there  may  be  cited  the  number 
of  references  to  scientific  publications  made  by  Menzel  in 
footnotes.  Before  1923,  references  were  made  to  18  arti- 
cles; in  the  years  1923-25  there  were  22  references;  while 
in  the  interval  1926-30,  no  less  than  100  publications 
appeared,  of  which  number  there  were  69  citations  in  1928 
and  1929. 

Following  the  eclipse  of  1908  observed  by  Campbell,  the 
next  eclipse  to  provide  a  photograph  of  the  flash  spectrum 
was  1914,  when  Abetti  secured  spectra  of  small  dispersion. 
At  the  eclipse  of  1918,  on  account  of  widespread  clouds,  the 
only  photograph  of  the  flash  spectrum  in  good  detail  was 
obtained  by  H.  C.  Wilson  with  concave  grating,  but  also  with 
small  dispersion.  No  attention  was  paid  to  the  spectrum  of 


280  ECLIPSES    OF    THE    SUN 

the  chromosphere  in  1919  and  1922,  the  Einstein  problem 
being  of  paramount  importance.  Little  progress  was  made 
at  the  1923  eclipse. 

At  the  1925  eclipse,  Curtis  and  Burns1  with  a  concave 
grating  of  short  focal  length  obtained  photographs  at  the  red 
end  of  the  spectrum  on  plates  stained  with  dicyanine.  About 
100  lines  were  measured  between  the  D  lines  and  8807  A, 
the  dispersion  being  small,  of  80  angstroms  per  mm.  At  this 
same  eclipse,  Anderson  with  a  21 -foot  concave  grating  with- 
out slit  secured  spectra  on  which  the  definition  was  vitiated 
by  the  poor  qualities  of  seeing.  At  the  same  eclipse,  with  10- 
foot  concave  grating  without  slit,  Mitchell  obtained  photo- 
graphs of  the  flash  spectrum.  For  publication,  the  results 
were  combined  with  those  of  the  1905  eclipse  photographed 
with  the  same  equipment.  At  the  extreme  ultra-violet,  the 
measures  of  the  1926  spectra  of  Davidson  and  Stratton  were 
included  in  the  discussion  by  Mitchell.  This  publication, 
like  that  of  Menzel,  involved  the  newer  ideas  of  the  structure 
of  the  atom.  In  fact,  both  Menzel  and  Mitchell,  through  the 
kindness  of  the  Mount  Wilson  astronomers,  had  free  access 
to  the  valuable  compilations  on  multiplet  structure.  The 
region  of  wave-lengths  included  in  Mitchell's  discussion  was 
from  3066  to  7065  A,  while  that  of  Menzel  included  the  re- 
gion 3229  to  5328  A. 

In  1926,  Davidson  and  Stratton  "  secured  beautiful  photo- 
graphs with  a  quartz  slit-spectrograph  giving  excellent  defi- 
nition from  3066  to  3968  A.  With  a  camera  of  i9-feet  focus 
and  direct-vision  prism,  the  region  photographed  was  ex- 
tended to  H(3,  and  with  camera  of  4O-feet  focus  and  prism 
of  40°,  the  region  was  further  extended  to  beyond  Ha.  The 
photographs  with  the  two  prismatic  cameras  were  taken  with- 
out slit  and  furnish  heights  for  the  stronger  lines  of  the  spec- 
trum. The  results  were  grouped  according  to  multiplets 
shown  by  the  different  chemical  elements. 

Davidson  and  Stratton  compared  the  heights  derived  from 
their  prismatic  cameras  with  those  of  the  1905  eclipse  with 
concave  grating.  Differences  were  noted  which  depend 

1  Publications  of  Allegheny  Observatory,  6,  95,  1925. 

2  Memoir  R.  A.  S.,  64,  105,  1927. 


DISCUSSING    THE    FLASH    SPECTRUM          281 

mainly  on  definition  but  there  was  no  systematic  error  in 
zero-point  between  the  heights  from  the  two  eclipses. 

At  the  1927  eclipse,  Pannekoek  and  Minnaert  secured  ex- 
cellent photographs  of  the  flash  spectrum  at  Gallivare  in 
Lapland,  one  of  the  few  places  where  clear  skies  greeted  the 
observers  of  this  eclipse.  The  spectrograph  was  one  of  three 
prisms  and  slit,  the  photograph  of  the  first  flash  was  in  splen- 
did definition  between  the  wave-lengths  4154  and  4751  A. 
The  dispersion  of  i  mm  equal  to  3  angstroms  is  the  largest 
ever  successfully  employed  on  eclipse  spectra. 

The  most  interesting  part  of  their  discussion  is  the  pho- 
tometry of  the  flash  spectrum,  the  Moll  registering  micropho- 
tometer  being  used  for  the  measurement  of  intensities.  In 
spite  of  the  large  dispersion  employed,  no  attempts  were 
made  to  measure  exact  wave-lengths.  The  positions  of  the 
spectral  lines  were  read  from  the  photometer  sheets  and 
wave-lengths  were  derived  from  a  Hartmann  formula.  Com- 
parisons were  made  for  each  line  with  Rowland  and  with  the 
intensities  estimated  by  Mitchell  from  the  1905  eclipse,  an 
excellent  agreement  being  found  between  the  1905  and  1927 
results. 

These  spectra  were  very  carefully  calibrated  by  means  of 
three  other  plates  cut  from  the  same  larger  photographic 
plate  as  the  one  on  which  the  flash  spectrum  was  photo- 
graphed, and  developed  together  with  it.  On  one  of  these 
plates  was  photographed  the  iron  arc;  on  the  second,  the  con- 
tinuous spectrum  of  a  standard  incandescent  lamp  whose  in- 
tensity distribution  over  various  wave-lengths  was  known; 
and  on  the  third,  the  Fraunhofer  spectrum  of  the  sun.  In 
front  of  the  slit  of  the  spectroscope  they  had  placed  a  "  step 
weakener."  By  means  of  this  they  obtained  a  calibration 
curve  for  each  region  of  the  spectrum  separately;  and  by 
means  of  the  spectrum  of  the  standard  lamp  and  the  Fraun- 
hofer spectrum  they  obtained  the  relation  between  these  in- 
dividual calibration  curves. 

In  a  very  thorough  discussion  they  were  able  to  find  the 
number  of  ergs  per  second  emitted  in  a  unit  solid  angle  by 
an  arc  of  the  chromosphere  i'  in  length  corresponding  to  each 
bright  line  of  their  flash  spectrum.  Combining  this  material 


282  ECLIPSES    OF    THE    SUN 

with  the  theoretical  intensities  of  lines  within  multiplets, 
they  made  a  careful  study  of  the  effects  of  self-reversal  in  the 
flash  spectrum.  They  also  found  the  total  intensities  of 
entire  multiplets  relative  to  the  total  intensities  of  other  mul- 
tiplets  of  the  same  element.  When  the  theory  of  atomic 
spectra  has  been  sufficiently  developed  to  predict  the  relative 
intensities  from  one  multiplet  to  another,  such  absolute  in- 
tensities in  the  chromospheric  spectrum  will  offer  the  best 
data  for  comparison,  much  better,  in  fact,  than  laboratory 
spectra. 

To  derive  from  the  measured  transmissions  of  the  micro- 
photometer,  a  knowledge  of  the  absolute  units  of  energy  of 
the  source  in  the  chromosphere  is  a  problem  beset  with  many 
difficulties.  By  the  employment  of  their  careful  methods  of 
calibration,  this  was  accomplished  in  three  steps:  (i)  From 
the  measured  transmission  curve  for  a  short  interval  of 
wave-lengths  to  ascertain  the  apparent  intensity.  (2)  By 
proper  allowance  for  instrumental  causes,  such  as  change  of 
dispersion  in  the  spectrograph,  color  sensitivity  of  the  pho- 
tographic plate,  selective  absorption  in  the  apparatus,  and 
other  factors,  to  change  the  apparent  intensities  thus  deter- 
mined into  real  intensities.  (3)  By  proper  methods  of  cali- 
bration and  standardization  it  is  then  necessary  to  find  the 
absolute  intensities.  According  to  Pannekoek  and  Minnaert, 
"  It  is  easy  to  foresee  that  the  determination  of  the  apparent 
intensities  is  the  most  accurate,  that  the  determination  of 
the  real  intensities  is  more  difficult,  and  that  the  determina- 
tion of  absolute  intensities  may  be  liable  to  many  sources  of 


errors." 


After  making  allowance  for  all  possible  factors  and  deriv- 
ing intensities  measured  in  absolute  units,  comparisons  were 
then  made  with  Rowland's  intensities  and  with  those  of 
Kayser.  An  interpretation  of  these  comparisons  which  will 
be  reasonably  free  from  large  systematic  errors  is  encum- 
bered with  many  grave  difficulties  for  the  reason  that  the  ab- 
solute scale  of  intensities  derived  from  the  microphotometer 
has  no  definite  relationship  with  the  estimates  of  Row- 
land and  Kayser  which  are  on  arbitrary  scales.  On  account 
of  the  many  lines  of  the  iron  spectrum,  this  element  gave  the 


DISCUSSING    THE    FLASH    SPECTRUM          283 

most  complete  information.  Between  the  microphotometer 
readings  and  Kayser's  estimates  there  was  found  a  strong 
dependence  on  wave-length;  a  line  at  4200  A  having  a  three- 
fold greater  microphotometer  intensity  than  a  line  of  similar 
Kayser  intensity  at  4700  A.  On  the  other  hand,  a  close 
agreement  was  found  to  exist  between  Rowland's  solar  in- 
tensities and  Kayser's  estimates  for  Fe;  but  a  similar  agree- 
ment is  not  found  for  other  important  elements  in  the  sun, 
such  as  Cr,  Mn,  Tiy  Ni  and  Co,  the  Rowland's  intensities  be- 
ing much  lower  than  Kayser's  intensities  of  the  elements. 

In  the  calibration  of  Rowland's  scale,  referred  to  in  a  later 
chapter,  Russell,  Adams  and  Miss  Moore  find  that  the  Row- 
land scale  at  4200  A  is  seven  per  cent  greater  than  at  4700  A, 
a  result  not  in  harmony  with  the  findings  from  the  1927 
eclipse.  On  account  of  this  disagreement  it  is  evident,  either 
that  the  1927  spectra  were  not  properly  calibrated  or  else 
that  the  homogeneous  scale  derived  at  Mount  Wilson  in  the 
process  of  the  revision  of  Rowland's  Table  must  be  changed 
by  a  large  factor. 

As  a  further  result  of  their  spectra,  Pannekoek  and  Min- 
naert  find  that,  "  The  energy  emitted  by  the  whole  chromo- 
sphere in  each  of  these  wave-lengths  is  not  proportional  to 
the  theoretical  emission.  In  general,  the  observed  intensities 
increase  together  with  the  theoretical  intensities,  but  more 
slowly.  This  proves  that  the  chromosphere  may  not  be  con- 
sidered as  an  optically  thin  layer  of  small  effective  depth,  but 
that  there  is  an  appreciable  amount  of  self-absorption  in  the 
layer  of  gas  viewed  tangentially."  Further  discussions  car- 
ried out  by  Pannekoek  1  lead  to  the  conclusion  that  the  density 
of  the  chromospheric  gases  being  small,  the  effect  of  colli- 
sions may  be  neglected  with  the  result  that  absorption  and 
emission  of  radiation  is  the  only  method  of  energy  transfer. 
The  atoms  forming  the  chromosphere  being  exposed  to  the 
radiation  from  the  hemisphere  below,  there  is  no  diminution 
of  radiation  at  the  highest  levels.  This  implies  that  the  chro- 
mospheric gases  produce  only  a  small  part  of  the  total  ab- 
sorption in  the  solar  chromosphere  with  the  consequent  result 
that  the  Fraunhofer  absorption  takes  place  almost  com- 

i  B.  A.  N.,  4,  263,  1928. 


284  ECLIPSES    OF    THE    SUN 

pletely  within  the  confines  of  the  lower  levels  of  the  reversing 
layer.  These  results  lead  to  the  conclusion  that  the  chromo- 
spheric  gases  are  supported  mainly  by  radiation  pressure. 
For  obvious  reasons,  these  conclusions  must  be  accepted  with 
reservations. 

By  combining  the  material  from  six  successive  exposures 
they  were  able  to  find  the  density  gradient  of  hydrogen  in 
the  chromosphere  up  to  heights  of  3000  km.  They  found  that 
its  density  gradient  was  much  less  than  would  be  expected  in 
an  isothermal  atmosphere  in  which  the  only  counterbalance 
to  gravitation  is  gas  pressure.  By  the  same  method  they 
found  that  the  density  of  the  emitting  atoms  of  helium  is 
actually  greater  at  700  km  above  the  photosphere  than  at 
levels  closer  to  the  photosphere.  Their  work  is  the  most  suc- 
cessful yet  accomplished  in  exact  spectrophotometry  of  the 
chromosphere. 

The  region  investigated  at  the  1927  eclipse,  of  600  ang- 
stroms at  the  red  side  of  4153  A,  is  but  a  small  portion  of  the 
chromospheric  spectrum.  Menzel  (op.  cit.,  p.  12)  with  the 
Moll  registering  microphotometer  measured  the  1905  Lick 
moving-plate  spectrum  between  3913  and  4466  A,  the  region 
of  best  definition.  Runs  were  made  in  the  ordinary  way  at 
six  different  levels,  assumed  to  be  at  170,385,600,  1030, 1675, 
and  2030  kilometers,  respectively,  above  the  photosphere. 

Although  the  method  adopted  by  Menzel,  developed  in- 
dependently, for  reducing  the  microphotograms,  was  very 
similar  to  that  of  Pannekoek  and  Minnaert,  nevertheless  he 
worked  under  severe  handicaps  in  that  he  did  not  even  know 
the  character  of  the  photographic  plate  used  at  the  1905 
eclipse,  and  naturally,  no  standard  squares  were  impressed 
thereon  for  purposes  of  calibration.  No  corrections  were 
made  by  him  for  atmospheric  or  instrumental  absorption. 

As  stated  in  the  preceding  chapter,  the  slot  of  the  moving 
plate  permitted  an  integrated  radiation  of  300  kms  in  each 
spectral  line,  and  moreover,  each  line  under  consideration 
was  defined  on  one  side  by  the  serrated  edge  of  the  moon, 
and  on  the  other  side  by  emitting  atoms  assumed  to  lie  above 
the  six  different  levels  under  consideration. 

With  so  many  unknown  factors,  and  with  heights  subject 


THE  APPROACHING  SHADOW  OF  THE  MOON,  BAKER,  OREGON,  JUNE  8,  1918 
From  the  painting  by  Howard  Russell  Butler,  N.  A. 


DISCUSSING    THE    FLASH    SPECTRUM          285 

to  systematic  errors,  Menzel  is  forced  to  evaluate  his  material 
from  the  interagreement  of  results  from  separate  lines. 
"When  the  continuous  background  was  subtracted,  it  was 
found  that  all  of  the  curves  for  a  given  chromospheric  height 
required  multiplication  by  a  single  factor,  inversely  propor- 
tional to  the  ordinate  at  maximum,  to  bring  them  into  sur- 
prisingly good  agreement.  This  was  probably  due,  in  part, 
to  the  short  spectral  region  studied.  No  correction  for  atmos- 
pheric or  instrumental  absorption  has  been  introduced.  The 
intensity  of  a  spectral  line  is  equal  to  the  area  under  the  '  true 
intensity  '  curve.  Since  all  of  the  curves,  for  a  given  level, 
were  of  similar  '  shape,'  the  ordinate  at  the  mid-point  deter- 
mines the  relative  intensity  of  a  line. 

"  The  observed  peculiarity  of  line  shape  is  just  the  reverse 
of  what  might  have  been  expected  on  a  priori  grounds.  It  is 
easily  shown  that  the  widening  that  arose  from  the  deviation 
of  the  small  segment  of  the  solar  crescent  from  a  straight  line 
was  negligible.  Almost  all  other  causes  that  act  to  broaden 
a  spectral  line  —  pressure,  Stark  effect,  self-reversal,  tem- 
perature ( Doppler  motion )  —  apparently  should  be  more 
effective  at  the  lower  levels.  There  is,  of  course,  the  chance 
that  '  seeing  '  conditions  during  totality  were  not  uniform, 
but  there  is  evidence  against  this  explanation,  in  the  form  of 
a  marked  exception  to  my  previous  statement  that  all  lines, 
at  a  given  level,  conform  to  the  same  contour  curve.  The 
line  4026  A  of  helium  is  essentially  a  i  high-level '  line,  as  is 
immediately  evident  from  its  great  extension  in  relation  to 
its  intensity.  Consequently,  most  of  the  intensity,  recorded 
on  the  tracings  at  lower  levels,  is  the  integrated  light  from 
much  greater  heights.  The  lines  of  greatest  intensity  pre- 
sent a  special  problem.  Small  errors  in  drawing  the  char- 
acteristic curve  of  the  photographic  plate  produce  large  er- 
rors in  the  resultant  intensities.  Rather  than  introduce  such 
indeterminate  intensities,  I  have  computed  the  intensities  of 
these  lines  on  the  assumption  that  their  contours  were  the 
same  as  for  the  fainter  lines.  It  then  suffices  to  determine 
the  ordinate  at  any  point  of  the  line  contour.  The  intensity 
is  measured,  not  at  the  center  of  the  line,  but  in  the  wings 
where  the  photographic  density  is  less,  and  where  the  char- 


286  ECLIPSES    OF    THE    SUN 

acteristic  curve  is  more  certain.  Intensities  calculated  in  this 
manner  are '  upper  limits.'  They  are  subject  to  correction  for 
sel  f  -r  ever  sal. " 

Continuing  the  study  by  the  investigation  of  the  difficult 
problem  of  self -reversal,  Menzel  continues  on  page  261  as 
follows.  "The  density  gradient  of  the  chromosphere  is 
closely  related  to  the  observed  intensities  of  the  chromo- 
spheric  lines  at  the  various  levels  studied  by  the  micropho- 
tometer.  The  energy  at  any  one  level  on  the  plate  represents 
the  integrated  light  from  all  the  higher  levels  not  yet  cut  off 
by  the  moon's  limb.  The  presence  of  self -reversal  compli- 
cates the  problem.  Theoretical  estimates  of  the  amount  of 
self-reversal  seem  to  be  unreliable.  We  can,  however,  de- 
termine the  amount  of  the  self-reversal  in  what  seems  to  be 
a  perfectly  direct  way,  by  comparing  the  observed  and  theo- 
retical intensities  through  the  intermediary  of  the  estimated 
flash  intensities." 

After  going  through  an  intricate  theoretical  discussion, 
Menzel  apparently  puts  little  weight  upon  his  final  results. 
This  is  shown  by  the  fact  that  in  the  determination  of  the 
distribution  of  vapors  in  the  chromosphere  and  the  deriva- 
tion of  density  gradients,  he  prefers  the  density  gradients  de- 
duced from  a  discussion  of  the  observed  heights  to  those  de- 
rived from  his  microphotometer  readings. 

One  of  the  most  pressing  needs  concerning  the  atom  at  the 
present  time  (1932)  is  a  more  accurate  knowledge  of  the 
contours  of  lines  in  the  ordinary  solar  spectrum  in  order  to 
ascertain  the  numbers  of  atoms  effective  in  producing  ab- 
sorption in  the  centers  and  wings  of  individual  lines.  Valu- 
able work  has  been  done  by  Unsold  and  others,  but  this  forms 
merely  the  beginning  of  a  more  comprehensive  research  nec- 
essary. In  the  Fraunhofer  spectrum  the  atoms  are  absorb- 
ing energy,  in  the  chromospheric  spectrum  they  are  emitting. 
To  those  who  are  not  fully  acquainted  with  the  difficulties 
involved,  it  would  seem  an  easy  matter  to  photograph  the 
chromospheric  spectrum  at  different  levels,  and  then  to  ob- 
tain a  knowledge  of  the  number  of  atoms  involved  at  differ- 
ent levels.  After  several  years  of  investigation  on  the  Fraun- 
hofer spectrum,  with  huge  equipment  and  unlimited  re- 


DISCUSSING    THE    FLASH    SPECTRUM  287 

sources,  we  do  not  yet  feel  on  very  firm  ground.  If  an  eclipse 
came  every  day,  and  if  photographs  could  be  obtained  of  the 
flash  spectrum  with  powerful  equipment,  that  is,  with  large 
dispersion  and  large  solar  image,  much  might  be  accom- 
plished. Unfortunately,  eclipses  are  rare  phenomena,  the 
"  flash  "  lasts  but  a  few  seconds,  and  it  is  impossible  to  use 
large  dispersion  with  a  large  solar  image  on  account  of  the 
paucity  of  light  and  erratic  conditions  of  seeing.  The  pres- 
ent writer  is  always  an  optimist  (or  he  would  not  be  an  eclipse 
observer)  but  he  has  a  practical  turn  of  mind.  In  the  im- 
mediate future,  he  cannot  foresee  any  brilliant  discoveries 
from  the  measurement  of  eclipse  spectra  by  the  micropho- 
tometer. 

A  knowledge  of  conditions  in  the  sun's  atmosphere  are  of 
such  great  importance  to  solar  physics  that  attempts  are 
made  to  observe  or  photograph  the  chromospheric  spectrum 
at  times  when  there  is  no  total  eclipse.  This  is  possible  either 
during  a  partial  eclipse  by  observations  at  the  cusps,  or  when 
there  is  no  eclipse  at  all.  Observations  were  secured  visually 
by  Fowler  in  London  during  the  progress  of  the  partial  eclipse 
of  the  sun,  April  17,1912.  At  its  maximum  phase,  this  eclipse 
was  0.91  total  (the  sun's  diameter  =  i.o).  The  6-inch  equa- 
torial was  employed  in  conjunction  with  the  Evershed  spec- 
troscope, the  observing  conditions  being  very  favorable.  The 
number  of  bright  chromospheric  lines  that  were  seen  vastly 
exceeded  expectations.  As  early  as  thirty-five  minutes  be- 
fore maximum  eclipse,  high-level  chromospheric  lines  were 
noted  at  the  cusps  and  the  number  of  lines  increased  with  the 
progress  of  the  eclipse.  During  the  maximum  phase,  hun- 
dreds of  Fraunhofer  lines  were  reversed,  in  fact  the  number 
of  lines  seen  was  so  great  that  it  was  impossible  to  record  all 
of  the  lines.  The  appearance  greatly  resembled  the  flash 
spectrum  that  had  already  been  observed  by  Fowler  at  more 
than  one  total  eclipse.  About  seventy  bright  lines  between 
b  and  D  were  actually  identified.  That  so  many  lines  of  the 
chromosphere  were  visible  near  the  cusps  at  the  time  of  the 
eclipse  may  be  partially  explained  as  the  result  of  reduced 
sky  illumination  due  to  the  fact  that  but  nine  per  cent  of  the 
sun  remained  uncovered.  A  greater  advantage  may  have 


288  ECLIPSES    OF    THE    SUN 

resulted  from  the  smaller  effect  of  unsteadiness  or  "  boiling  " 
at  the  cusp  as  compared  with  the  limb  under  ordinary  con- 
ditions. 

The  success  of  these  observations  was  so  great  that  it 
seemed  possible  to  Fowler  that  even  better  results  could  be 
obtained  at  a  similar  eclipse  by  the  use  of  more  powerful 
apparatus,  and  that  the  multitude  of  bright  lines  seen  visu- 
ally for  half  an  hour,  while  the  eclipse  ranged  from  eight-  to 
nine-tenths  total,  might  profitably  be  photographed  by  suit- 
ably arranged  instruments.  In  fact,  at  this  same  eclipse, 
Newall  at  Cambridge  actually  secured  successful  photo- 
graphs. On  the  best  photograph  only  about  forty  lines  were 
recorded  as  bright,  many  of  them  exceedingly  narrow  with 
the  continuous  spectrum  quite  faint.  Apparently,  therefore, 
it  is  more  difficult  to  photograph  the  bright  lines  than  it  is 
to  observe  them  visually.  In  view  of  the  success  attained  in 
photographing,  Newall  came  to  the  conclusion  '  that  "  ex- 
ceedingly valuable  work  could  be  carried  out  with  an  instru- 
ment of  high  power  by  an  observer  who,  in  a  total  eclipse, 
stationed  himself  to  the  north  or  south  of  the  band  of  to- 
tality at  such  a  distance  that  the  maximum  phase  was  about 
0.99.  At  such  a  station,  detailed  observations  could  be  con- 
veniently made  with  a  much  more  leisurely  program  and  with 
far  greater  completeness  than  on  the  central  line." 

In  view  of  their  success  in  1912,  the  Cambridge  2  observers 
prepared  to  photograph  the  eclipse  of  192 1  with  the  McClean 
spectroscope  of  the  Solar  Physics  Observatory.  This  instru- 
ment uses  an  image  of  the  sun  168  mm  in  diameter,  the 
dispersion  being  caused  by  a  6-inch  plane  grating.  Two 
photographs  were  secured  with  excellent  definition  —  but  the 
total  number  of  bright  lines  visible  was  only  two,  due  to  hy- 
drogen, not  a  single  bright  metallic  line  appearing  on  either 
of  the  plates.  A  similar  disappointment  resulted  from  the 
photographs  taken  with  the  Huggins  refractor,  also  at  Cam- 
bridge. 

Observations  similar  in  kind  were  also  made 3  visually  at 

1  Monthly  Notices,  R.  A.  S.,  72,  538,  1912. 

2  Monthly  Notices,  R.  A.  S.,  81,  482,  1921. 

3  Monthly  Notices,  R.  A.  S.,  81,  485,  1921. 


DISCUSSING    THE    FLASH    SPECTRUM          289 

the  same  eclipse  by  Fathers  Cortie  and  Rowland  at  Stony- 
hurst.  A  Browning  spectroscope  with  a  dispersion  of  eight 
prisms  of  60°  was  employed,  the  region  observed  being  in 
the  green  from  5167  A  to  5400  A.  "  Every  line  in  the  field 
was  reversed,  the  bright  lines  tapering  to  a  point  indicating 
decrease  of  pressure." 

Newall  (loc.  cit.)  could  not  offer  any  explanation  for  his 
failure  to  photograph  the  bright  lines  at  the  1921  partial 
eclipse.  The  experience  of  the  writer,  both  at  total  eclipses 
and  at  Mt.  Wilson  (see  below)  causes  him  to  suspect  that  the 
conditions  of  seeing  at  Cambridge  were  inferior  to  what  they 
were  at  the  eclipse  of  1912  in  spite  of  the  fact  that  at  the 
time  the  observers  regarded  the  conditions  as  about  equal. 

A  quarter  of  a  century  ago,1  Hale  and  Adams  secured  pho- 
tographs of  the  flash  spectrum  without  an  eclipse  by  using  the 
6o-foot  tower  telescope  and  powerful  spectograph  attached. 
The  method  employed  was  to  allow  light  from  the  sun's  limb 
to  fall  upon  a  diagonal  prism  so  placed  as  to  reflect  the  light 
horizontally  to  a  second  prism  immediately  above  the  slit. 
The  first  prism  was  mounted  upon  a  slide  with  a  screw  ad- 
justment allowing  motion  toward  or  away  from  the  second 
prism. 

After  the  sun's  image  has  been  brought  to  the  slit,  the 
observer  selects  a  bright  line  of  the  chromospheric  spectrum 
and  brings  it  into  the  field  of  view  of  an  eye-piece,  mounted 
in  an  opening  near  the  end  of  the  plate-holder.  During  the 
exposure,  this  line  is  maintained  at  maximum  brightness  by 
guiding  with  the  screw  controlling  the  position  of  the  first 
diagonal  prism,  thus  moving  the  sun's  image  slightly  on  the 
slit.  The  second  order  spectrum  was  employed  giving  a  dis- 
persion of  i  mm  =  0.9  angstroms;  and  exposures  of  four 
minutes  were  required  in  the  yellow  part  of  the  spectrum, 
and  double  this  amount  in  the  red. 

Two  reports  on  the  wave-lengths,  etc.,  obtained  from  the 
spectra  have  been  published  from  Mt.  Wilson.  The  first  one 
by  Hale  and  Adams  appeared  in  Communications  No.  41, 
and  the  second  by  Adams  and  Burwell  in  Communications 
No.  95,  the  former  coming  before  and  the  latter  after  the 

1  Astrophysical  Journal,  30,  222,  1909;  Mt.  Wilson  Contr.,  No.  41. 


290  ECLIPSES    OF    THE    SUN 

publication  by  Mitchell  of  his  flash  spectrum  results  obtained 
at  the  eclipse  of  1905.  For  obvious  reasons,  the  second  com- 
munication from  Mt.  Wilson  is  more  complete  than  the  first. 
Unquestionably  the  photographs  at  Mt.  Wilson  were  secured 
at  a  very  low  level,  but  whether  at  a  lower  level  than  the 
1905  eclipse  spectra  it  is  rather  difficult  to  decide.  In  com- 
paring the  results  obtained  within  and  without  an  eclipse, 
there  are  very  great  advantages  in  favor  of  the  latter  method, 
the  most  important  being  the  possibility  of  securing  a  much 
higher  dispersion  by  the  method  without  an  eclipse  than  can 
be  secured  at  the  time  of  an  eclipse.  The  dispersion  of  the 
1905  eclipse  spectra  was  i  mm  =10.9  angstroms,  or  one- 
twelfth  of  that  in  the  discussion  in  the  Mt.  Wilson  Com- 
munications. For  reasons  stated  in  the  foregoing,  it  is  im- 
possible to  use  large  dispersion  at  an  eclipse.  Because  the 
exposures  without  an  eclipse  can  be  increased  at  will,  the  dis- 
persion can  also  be  increased,  and  it  may  be  possible  for  the 
Mt.  Wilson  observers  to  employ  the  i5o-foot  tower  tele- 
scope with  the  7  5 -foot  spectograph  in  the  second  order  and 
thus  secure  a  dispersion  thirty  times  that  of  the  1905  eclipse 
spectra.  At  an  eclipse  only  two  exposures  of  the  flash  spec- 
trum are  possible,  and  unfortunately  the  best  focus  is  se- 
cured more  or  less  only  as  the  result  of  a  happy  accident. 
Outside  of  an  eclipse  there  are  no  such  limitations,  observa- 
tions may  be  repeated  until  moments  of  good  seeing  are  se- 
cured and  until  photographs  are  obtained  of  the  best  defini- 
tion exhibiting  the  results  of  layers  sufficiently  close  to  the 
sun. 

The  comparison  of  the  results  obtained  with  and  without 
an  eclipse  may  be  briefly  summarized  here.  First,  let  us 
take  up  the  number  of  the  lines  photographed  by  the  two 
methods  in  a  given  region  of  the  spectrum.  Compared  with 
spectra  taken  outside  of  an  eclipse,  those  obtained  at  an 
eclipse  have  the  two-fold  disadvantage  of  the  much  smaller 
dispersion  and  of  the  superposition  of  the  eclipse  lines  on 
a  very  strong  continuous  spectrum.  In  the  process  of  meas- 
uring the  eclipse  spectra,  it  was  difficult  to  differentiate  the 
weakest  lines  from  the  continuous  spectrum.  Consequently 
many  eclipse  lines  were  actually  measured  in  the  flash  spec- 


DISCUSSING    THE    FLASH    SPECTRUM          291 

trum  but  were  not  published  at  the  time  (see  Astro  physical 
Journal,  38,  481,  1913),  for  it  seemed  unwise  to  add  any  lines 
to  the  publication  unless  those  lines  could  be  definitely  identi- 
fied with  lines  appearing  in  Rowland's  Tables.  For  these 
reasons,  therefore,  it  must  be  concluded  that  the  number  of 
the  weakest  lines  secured  at  Mt.  Wilson  without  an  eclipse 
is  probably  not  greater  than  the  number  found  on  Mitchell's 
eclipse  spectra.  Since  the  intensities  of  the  lines  depend  on 
the  levels  at  which  the  lines  originate,  it  seems  highly  prob- 
able, therefore,  that  the  levels  photographed  within  and  with- 
out an  eclipse  are  not  very  different.  For  the  stronger  lines 
originating  at  higher  levels,  the  intensities  inside  and  outside 
of  the  eclipse  greatly  differ,  such  variations  however  being 
readily  explained  on  account  of  these  very  differences  in  level. 

Regarding  the  accuracy  of  the  wave-lengths,  the  following 
may  be  said.  On  account  of  the  spreading  of  the  photographic 
images  of  the  strong  lines  of  the  eclipse  spectra  it  is  im- 
possible to  secure  accurate  wave-lengths  from  such  lines. 
Omitting  these  lines,  the  eclipse  wave-lengths  differ  from 
Rowland  on  the  average  by  0.020  angstroms.  With  the 
twelve-fold  greater  dispersion  at  Mt.  Wilson  the  wave- 
lengths have  an  accuracy  of  0.012  angstroms,1  being  a  pre- 
cision about  twice  that  obtained  from  eclipse  spectra. 

With  a  slit  tangent  to  the  sun's  limb  it  is  evident  that  the 
length  of  the  lines  in  the  spectrum  taken  without  an  eclipse 
should  furnish  information  regarding  the  levels  at  which 
such  lines  originate.  Such  measures  have  not  as  yet  been 
carried  out  at  Mt.  Wilson.  The  need  of  more  reliable  deter- 
minations of  the  levels  at  which  the  spectrum  lines  of  dif- 
ferent intensities  originate  and  the  great  importance  of  such 
knowledge  in  present-day  problems  of  solar  physics  cannot 
be  overemphasized. 

In  summing  up  the  problem  regarding  the  photography  of 
the  flash  spectrum  as  it  appeared  to  him  in  1924  while  in  resi- 
dence for  a  brief  time  at  Mt.  Wilson  Observatory,  the  writer 
voiced  his  opinions  in  the  following  paragraph  which  is  found 
in  the  second  edition. 

The  recommendation  is  therefore  made  to  the  investiga- 

1  Mt.  Wilson  Contributions,  No.  95. 


292  ECLIPSES    OF    THE    SUN 

tors  of  solar  rotation,  and  is  hereby  urged  upon  them  for 
their  consideration,  that  they  curtail  their  work  on  solar 
rotation,  and  instead  devote  their  energies  for  a  short  while 
to  the  flash  spectrum  without  an  eclipse.  With  the  equip- 
ment already  in  hand,  and  under  good  conditions  of  seeing 
and  with  adequate  facilities  for  proper  guiding,  good  photo- 
graphs of  the  flash  spectrum  should  be  possible  at  very  low 
levels.  The  accurate  measurement  of  wave-lengths,  and  par- 
ticularly the  determinations  of  the  levels  at  which  different 
lines  of  the  spectrum  originate,  will  add  greatly  to  our  knowl- 
edge concerning  these  lines  and  will  supply  information  so 
sadly  needed  in  deciphering  the  laws  underlying  the  produc- 
tion of  spectral  lines.  The  Bohr  atom  and  Saha's  theory  of 
ionization  have  made  possible  the  identification  of  new 
spectral  lines  in  the  sun  and  in  the  laboratory,  and  undoubt- 
edly we  are  on  the  eve  of  a  very  great  advance  in  knowl- 
edge regarding  the  laws  underlying  the  production  of  lines 
in  the  spectra  of  various  sources,  provided  wave-lengths 
of  great  accuracy  and  free  from  systematic  errors  can  be 
furnished. 

The  eight  hectic  years  since  the  second  edition  of  this  book 
have  revolutionized  our  conceptions  of  solar  research.  Re- 
garding the  problem  of  the  flash  spectrum,  Director  Adams 
gave  the  writer,  while  he  was  in  residence  at  Mt.  Wilson, 
every  opportunity  for  observing  with  the  i5o-foot  tower  tele- 
scope. The  writer  then  realized  the  important  role  played 
by  the  qualities  of  seeing  and  steadiness  of  image  of  the  sun 
on  the  slit  of  the  spectrograph.  If  the  slit  is  set  tangent  to 
the  limb  of  the  sun  and  if  the  bright-lined  spectrum  is  ob- 
served visually  by  the  methods  described  above,  then  it  is 
immediately  noticeable  that  if  the  seeing  is  ragged  or  fair, 
there  will  be  few  bright  lines  that  can  be  seen,  such  for  in- 
stance as  the  stronger  lines  of  the  solar  spectrum  like  the 
6-group  in  the  green.  If  the  seeing  improves  and  the  image 
of  the  sun  becomes  more  and  more  steady,  then  more  and 
more  lines  are  seen  reversed  into  bright  lines.  At  Mt.  Wil- 
son, the  best  seeing  ordinarily  comes  shortly  after  sunrise. 
After  the  sun's  rays  heat  up  the  slopes  of  the  mountain  and 
the  rising  of  the  heated  air  disturbs  the  atmosphere,  the 


DISCUSSING    THE    FLASH    SPECTRUM          293 

steadiness  of  the  image  of  the  sun  on  the  slit  of  the  spectro- 
graph  becomes  less  and  less.  In  the  short  time  at  his  disposal 
for  making  observations,  the  writer  came  to  the  conclusion 
that  for  photographing  the  flash  spectrum  without  an  eclipse 
the  best  possible  conditions  are  absolutely  necessary,  or  in 
other  words,  with  seeing  and  steadiness  of  quality  10  where 
10  represents  the  maximum  of  perfection.  If  therefore  one 
wishes  to  obtain  such  photographs  without  an  eclipse,  he 
must  be  continually  on  the  alert  to  catch  the  few  precious 
moments  of  perfect  seeing  when  they  come;  and  then  if  not 
too  impatient,  after  taking  hundreds  or  possibly  thousands 
of  photographs  for  this  purpose,  a  photograph  may  finally  be 
secured  which  will  be  of  quality  equal  to,  or  even  better  than, 
that  of  Adams  obtained  in  the  early  years  at  Mt.  Wilson. 
These  observations  only  serve  to  emphasize  the  importance 
of  excellent  seeing,  no  matter  what  method  is  employed  for 
photographing  the  flash  spectrum. 

Anticipating  somewhat  the  theoretical  considerations  to  be 
developed  in  the  three  following  chapters,  it  is  possible  to 
make  a  summary  of  results  achieved  from  a  study  of  the 
flash  spectrum,  and  also  to  make  recommendations  for  work 
at  future  eclipses.  The  following  general  conclusions  may 
be  drawn: 

1.  On  account  of  the  different  conditions  under  which  the 
chromospheric  and  Fraunhofer  spectra  originate,  it  is  prob- 
able that  wave-lengths  from  the  two  spectra  differ  systemati- 
cally.   However,  these  differences  do  not  amount  to  as  much 
as  0.02  A. 

2 .  Every  strong  line  in  the  Fraunhofer  spectrum  is  found 
in  the  flash  spectrum,  and  every  strong  line  in  the  latter 
(with  the  exception  of  H  and  He  lines)  is  matched  by  a  line 
in  the  former. 

3.  The  flash  spectrum  may  therefore  be  regarded  as  a 
reversal  of  the  Fraunhofer  spectrum. 

4.  The  "  flash  "  is  not  an  instantaneous  appearance.    At 
the  beginning  of  totality  the  chromospheric  lines  of  greatest 
elevation  appear  first,  and  at  the  end  of  totality  remain  the 
last. 

5.  The  "  reversing  layer  "  which  contains  the  majority  of 


294  ECLIPSES    OF    THE    SUN 

the  low-level  lines  of  the  chromosphere  is  about  600  km  in 
height. 

6.  The  "  reversing  layer  "  has  no  existence  separate  from 
the  chromosphere. 

7.  It  is  the  densest  part  of  the  chromosphere  lying  closest 
to  the  photosphere,  and  it  is  the  cause  of  the  greatest  por- 
tion of  the  absorption  producing  the  Fraunhofer  lines. 

8.  The  "  Evershed-effect "  measured  in  sun-spots,   and 
photographs  of  flocculi  which  exhibit  vastly  different  aspects 
when  photographed  at  various  elevations  above  the  photo- 
sphere prove  that  the  shadings  of  such  strong  lines  as  H  and 
K  are  caused  by  absorption  at  different  levels  and  pressures 
above  the  photosphere. 

9.  The  chromospheric  spectrum  differs  greatly  from  the 
ordinary  solar  spectrum  in  the  intensity  of  the  lines. 

10.  The  Fraunhofer  spectrum  is  essentially  an  arc  spec- 
trum.   The  chromospheric  spectrum  more  closely  resembles 
the  spark  spectrum,  and  its  spectrum  corresponds  to  an 
"  earlier  "  type  than  that  of  the  sun. 

1 1 .  Especially  prominent  in  the  chromosphere  are  the  en- 
hanced lines. 

1 2 .  The  enhanced  lines  ascend  to  greater  elevations  above 
the  photosphere  than  do  the  ordinary  lines. 

13.  The  increased  elevations  cause  greatly  diminished 
pressures. 

14.  As  Saha  has  shown,  the  reduced  pressures  permit  the 
ionization  of  the  atom.    As  a  result,  the  lines  of  the  ionized 
atoms  are  specially  prominent  in  the  flash  spectrum.    The 
enhanced  lines  are  produced  by  the  ionized  atoms. 

15.  The  depth  of  the  chromosphere  is  not  constant. 
Recommendations  for  future  work  on  the  flash  spectrum 

may  be  briefly  summarized.  The  most  important  contribu- 
tion from  the  chromosphere  will  undoubtedly  come  from  the 
investigations  of  heights  and  intensities  of  the  spectral  lines 
in  order  to  gain  further  information  regarding  atomic  struc- 
ture. To  be  of  the  greatest  value  in  furthering  scientific 
research,  photographs  of  the  flash  spectrum  should  be  se- 
cured with  large  dispersion,  they  should  extend  as  far  to 
the  violet  and  as  far  to  the  red  as  possible,  the  definition 


DISCUSSING    THE    FLASH    SPECTRUM          295 

should  be  of  the  very  best  and  the  exposures  should  be 
timed  so  as  to  photograph  the  very  lowest  possible  levels. 
The  occupation  of  a  station  near  the  edge  of  the  moon's 
shadow  path  would  permit  relatively  long  exposures  on  the 
low-lying  layers  closest  to  the  sun's  pole.  Such  spectra  would 
afford  interesting  comparisons  with  those  taken  near  the  cen- 
tral line  of  totality  which  give  spectra  near  the  sun's  equator. 
Another  important  investigation  for  the  future  will  be  to 
make  comparisons  between  flash  spectra  taken  at  different 
phases  of  the  sun-spot  period.  If  we  are  to  judge  by  the 
changes  which  take  place  in  the  spectra  of  the  corona,  we 
should  expect  that  the  flash  would  be  richest  in  lines  at  times 
of  maximum  sun-spots.  The  eruptions  taking  place  near 
spot  zones  seem  to  elevate  the  low-lying  metallic  vapors.  If 
this  be  true,  it  would  be  natural  to  expect  that  if  it  were  pos- 
sible to  photograph  at  a  constant  level  above  the  photo- 
sphere in  securing  the  flash  spectrum  at  an  eclipse,  the  lines 
should  appear  to  be  of  greater  strength  at  sun-spot  maximum 
than  at  minimum. 

The  type  of  spectrograph  is  all  important.  Eclipses,  how- 
ever, are  rare  phenomena.  After  the  eclipse  of  1932,  which 
will  be  assiduously  observed,  the  total  eclipses  of  the  next 
two  decades  require  long  voyages  and  expensive  expeditions. 
Few  institutions,  except  those  like  the  Lick,  U.  S.  Naval  and 
Mt.  Wilson  Observatories  in  the  United  States,  and  the 
British  Joint  Permanent  Eclipse  Committee,  can  afford  the 
luxury  of  assembling  spectrographs  for  infrequent  use  on 
chromospheric  work.  Hence  most  astronomers  will  be  forced 
to  use  the  equipment  they  already  have,  others,  less  fortunate, 
must  borrow  their  apparatus. 

As  it  is  impossible  to  employ  a  dispersion  sufficiently  high 
to  permit  the  detection  of  systematic  differences  in  wave- 
lengths between  the  solar  and  chromospheric  spectra,  we 
come  to  the  conclusion  that  eclipse  wave-lengths  are  of  sec- 
ondary importance,  serving  as  they  do  merely  as  a  means  of 
accurately  identifying  the  origins  of  the  spectral  lines.  Al- 
though it  is  far  easier  to  get  good  definition  by  the  use  of  a 
slit  spectrograph,  each  observer  must  come  to  the  decision 
whether  to  try  the  easier  method  and  thereby  sacrifice  in- 


296  ECLIPSES    OF    THE    SUN 

formation  regarding  levels,  or  to  try  the  more  difficult  plan 
of  photographing  without  a  slit  in  order  to  attempt  to 
gain  knowledge  regarding  heights  which  are  now  about  the 
most  important  contribution  that  can  come  from  eclipse 
spectra. 

In  work  on  eclipses,  as  in  all  other  branches  of  scientific 
research,  continued  advances  must  be  made,  if  the  time  and 
energy  and  money  are  to  be  profitably  spent.  Just  because 
eclipses  are  rare  phenomena  is  no  reason  in  itself  why  an 
astronomer  without  eclipse  experience  can  expect  to  make  a 
scientific  contribution  of  value  unless  he  knows  the  subject 
thoroughly  and  has  a  problem  of  importance  to  attack. 

Prisms  have  advantages  over  gratings  in  their  greater 
light-gathering  power.  For  work  at  the  blue  end  of  the  spec- 
trum, prisms  may  be  the  best  means  of  securing  the  high 
dispersion  now  necessary.  On  account  of  erratic  conditions 
of  seeing  at  the  time  of  an  eclipse,  no  one  should  attempt  to 
increase  the  dispersion  by  increasing  the  focal  length  of  the 
camera  to  20  feet  or  more.  No  photographs  of  the  flash  spec- 
trum of  first  quality  have  yet  been  taken  by  this  type  of 
prismatic  camera.  In  the  future,  its  use  at  eclipses  should  be 
discontinued. 

It  is  possible  to  use  higher  dispersion  on  the  chromosphere 
than  has  been  employed  up  to  the  present.  We  badly  need 
photographs  at  the  extreme  violet  end  of  the  spectrum  with 
large  dispersion  in  the  region  which  cannot  be  reached  by 
glass  prisms,  and  hence  quartz  must  be  used. 

Except  for  the  light-gathering  power,  gratings  have  dis- 
tinct advantages  over  prisms.  At  the  red  end  the  prismatic 
spectrum  is  crowded  together.  To  get  high  dispersion,  many 
prisms  must  be  used.  The  grating  gives  a  "  normal  "  spec- 
trum and  a  greater  region  of  wave-lengths  in  good  definition. 
If  gratings  are  used,  they  should  be  concave  and  not  flat  grat- 
ings. Similarly,  larger  dispersion  than  that  employed  to 
date  may  be  used  profitably  with  concave  gratings.  To  in- 
crease the  dispersion  one  cannot  expect  to  have  more  lines 
per  inch  than  the  number  of  15,000  per  inch  which  have  been 
used.  The  only  other  means  is  to  work  in  the  second  order 
(if  it  is  possible  to  rule  gratings  specially  for  this  purpose) ,  or 


DISCUSSING    THE    FLASH    SPECTRUM          297 

to  increase  the  radius  of  the  grating  beyond  that  of  10  feet, 
the  maximum  so  far  successfully  used  at  eclipses.  As  Ander- 
son L  has  pointed  out,  if  the  seeing  is  not  of  the  best  at  the 
time  of  the  eclipse,  the  area  of  the  chromospheric  arcs  on  the 
photographic  plate  will  be  increased  with  increase  of  focal 
length,  with  the  consequent  diminution  in  the  intensity  of 
illumination.  As  a  result,  the  faint  chromospheric  arcs  will 
be  below  the  threshold  value  of  the  plate  and,  in  consequence, 
these  lines  will  be  conspicuous  by  their  absence.  To  over- 
come this  difficulty,  Anderson  put  into  practice  for  the  1926 
eclipse  the  use  of  a  21 -foot  concave  grating  of  15,000  lines 
per  inch,  but  the  sizes  of  the  chromospheric  arcs  were  di- 
minished by  the  employment  of  mirrors.  This  method  has 
been  tried  out  at  the  eclipses  of  1926,  1927  and  April,  1930, 
but  clouds  interfered  in  each  case.  It  will  be  tried  again  in 
1932. 

The  author  does  not  believe  that  anyone  will  be  brave 
enough,  or  perhaps  foolhardy  enough,  deliberately  to  place 
himself  outside  the  path  of  totality  in  order  to  photograph 
the  spectrum  of  the  chromosphere.  At  the  eclipse  of  1900, 
Evershed  found  himself  outside  the  eclipse  track  as  the  re- 
sult of  an  accident.  To  have  the  longer  time  of  observation 
by  observing  at  the  cusps,  it  will  be  necessary  to  await  the 
time  when  a  total  or  annular  or  large  partial  eclipse  passes 
near  an  observatory  where  there  is  a  powerful  equipment 
with  large  dispersion.  Observations  of  value  will  be  pos- 
sible only  under  the  condition  that  the  eclipse  is  more  than 
ninety-five  per  cent  total.  The  present  generation  of  active 
astronomers  will  not  see  such  conditions  come  to  pass  within 
their  lifetimes.  At  the  1932  eclipse,  the  physical  laboratory 
of  McGill  University  will  be  inside  the  southern  edge  of  the 
eclipse  path. 

If  progress  is  made  in  the  near  future  on  the  spectrum  of 
the  chromosphere  with  a  much  greater  dispersion  than  is 
possible  at  eclipses,  the  only  method  left  is  that  without  an 
eclipse.  Unfortunately,  this  work  demands  very  high  quali- 
ties of  seeing  that  practically  never  occur.  Hence,  taken  all 
in  all,  it  appears  probable  that  we  shall  have  to  rely  on  total 

1  Publications  A.  S.  P.,  38,  239,  1926, 


298  ECLIPSES    OF    THE    SUN 

eclipses  for  adding  to  the  information  already  available  on 
the  chromosphere. 

The  splendid  work  of  Adams  at  the  Mt.  Wilson  Observa- 
tory in  photographing  the  spectra  of  the  brightest  stars  of 
various  types  with  very  high  dispersion  will  supplement  the 
work  on  the  chromosphere  by  giving  information  on  series 
relationships  and  multiplet  groups  in  spectra.  The  work 
so  beautifully  inaugurated  with  the  powerful  spectrograph 
attached  to  the  loo-inch  mirror  may  be  carried  out  with 
greater  facility  when  the  gigantic  2oo-inch  telescope  is  put 
into  operation. 

At  the  present  time,  astronomers  are  devoting  much  atten- 
tion to  the  measures  of  intensities  by  means  of  the  micro- 
photometer.  In  the  foregoing,  the  author  has  tried  to  point 
out  some  of  the  difficulties  to  be  overcome  when  applying  pho- 
tometric measures  to  eclipse  spectra.  Unless  the  measures 
can  be  reduced  to  absolute  units,  fairly  free  from  systematic 
errors,  the  measured  intensities  will  have  a  reliability  little 
higher  than  estimates  by  a  skilled  observer.  In  all  of  the 
discussions  so  far  published,  the  intensities  of  the  chromo- 
spheric  lines  have  been  compared  with  Rowland's  values  and 
with  those  of  the  laboratory,  both  of  which  are  on  estimated 
scales.  Hence  when  reducing  eclipse  spectra,  each  observer 
must  decide  for  himself  whether  it  is  worth  while  both  to 
estimate  and  measure  chromospheric  intensities,  when  per- 
force all  comparisons  must  be  made  with  arbitrary  estimates. 
However,  it  is  very  desirable  that  each  eclipse  spectroscopist 
should  place  standard  squares  on  his  photographic  plates  so 
that  later  these  may  be  available  for  photometric  purposes  if 
desired. 

Fixed  or  moving  plates  may  be  used  for  the  flash  spectrum 
with  either  prisms  or  grating.  Those  who  disagree  with  the 
author  in  his  opinion  that  the  fixed  plate  gives  the  more  reli- 
able information  have  the  opportunity  of  using  the  moving 
plate  method.  At  the  1932  eclipse,  both  methods  will  have 
a  thorough  trial  for  there  will  be  many  American  and  Euro- 
pean parties  in  the  field  with  well-equipped  expeditions. 


CHAPTER  XVII 

THE  IMPORTANCE  OF  IONIZATION 

THE  discussion  of  the  results  of  the  flash  spectrum 
obtained  in  1905  has  shown  the  great  importance  of 
enhanced  lines,  or  those  due  to  the  ionized  atom, 
and  the  explanation  offered  by  the  writer  was  that  the  cause 
of  the  enhancement  was  due  to  the  great  heights  attained  by 
the  vapors  producing  the  enchanced  lines  and,  consequently, 
to  the  reduced  pressures  at  which  these  lines  were  found. 
It  was  also  pointed  out  that  at  the  time  of  an  eclipse  the 
light  from  the  sun  is  capable  of  reaching  us  only  in  a  direc- 
tion tangential  to  the  sun's  surface.  As  a  result,  a  beam  of 
light  from  the  bottom  of  a  layer  only  500  km  in  thickness 
would  encounter  no  less  than  20,000  km  of  emitting  atoms 
in  the  tangential  line  of  sight  before  getting  out  of  the 
shallow  layer.  Any  theory  that  can  explain  the  method 
whereby  atoms  are  ionized  will  be  of  the  very  greatest  im- 
portance in  all  problems  of  modern  astronomy.  Such  a 
theory  has  been  developed  by  Dr.  Megh  Nad  Saha.  The 
sun  is  the  nearest  of  the  fixed  stars,  and  on  account  of  its 
proximity  its  structure  may  be  examined  in  detail.  In  the 
sun  and  stars  are  found  high  temperatures,  very  minute 
pressures  and  electromagnetic  conditions  that  together  cause 
ionization  to  take  place  with  great  facility.  The  celestial 
laboratories  thus  opened  to  the  astronomer  have  given  him 
the  opportunity  of  supplementing  the  work  of  the  physicist 
and  chemist  in  terrestrial  laboratories  in  the  combined  attack 
on  atomic  structure. 

Accepting  the  correctness  of  the  Bohr-Sommerfeld  theory 
of  atomic  radiation,  and  assuming  that  the  general  laws  of 
thermodynamics  apply  equally  well  to  electrons  and  to  mole- 
cules of  gases,  Saha  has  been  able  to  calculate  the  degree 
of  ionization  that  takes  place  in  gases  under  different  con- 

299 


300  ECLIPSES    OF    THE    SUN 

ditions  of  temperature  and  pressure,  and  has  derived  formu- 
las which  can  readily  be  applied  to  conditions  existing  in  the 
atmosphere  of  the  sun  and  of  the  stars.  This  theory  ex- 
plains both  qualitatively  and  quantitatively  many  of  the 
features  observed  in  the  spectrum  of  the  sun  and  of  the  stars, 
and  it  likewise  finds  a  ready  application  in  laboratory  spectra 
under  conditions  when  enhanced  lines  appear. 

In  addition  to  the  original  papers  by  Saha,1  valuable  con- 
tributions have  been  made  by  Milne,2  by  Russell,"  and  also 
by  many  others.4  Assuming  that  the  decomposition  of  a 
molecule  or  an  atom  into  one  or  more  electrons  and  a  posi- 
tively charged  ion  is  essentially  of  the  same  nature  as  an 
ordinary  chemical  reaction,  Saha  derives  a  simple  equation 
to  express  the  self-ionization  of  a  gas  at  high  temperatures. 
The  equation  derived  is: 

P  x2 
^* 


I    —  X2 

x/(i  — x}  is  the  ratio  of  the  percentage  of  atoms  ionized 
to  those  left  neutral,  and  this  ratio  multiplied  by  the  partial 
pressure  of  the  free  electrons  (P  x/(i  +  x)  )  is  equal  to  Ky 
which  is  a  function  only  of  the  absolute  temperature  of  the 
gas  and  the  ionization  potential.  This  latter  is  a  measure 
of  the  work  done  to  ionize  a  single  molecule,  or  to  drive  an 
electron  from  its  neutral  ring  to  infinity,  and  it  is  expressed 
as  the  number  of  volts  through  which  the  electron  must 
fall  to  acquire  this  energy.  Since  the  ionization  potential  is  a 
constant,  the  quantity  K,  in  the  formula  above,  depends  only 
on  the  absolute  temperature.  Hence  for  a  given  pressure, 
the  smaller  the  ionization  potential  P,  the  more  nearly  x 
approaches  unity,  or  in  other  words  the  more  nearly  com- 
plete is  the  ionization.  For  all  gases  where  the  ionization 
potential  is  known,  Saha  is  enabled  to  calculate  the  per- 
centage of  ionization  found  under  different  conditions  of  tem- 
perature and  pressure.  The  higher  the  value  of  the  ionization 

1  Philosophical  Magazine,  40,  472   and   809,   1920;   Ibid.,  41,  267,    1921;  and 
Proceedings  of  the  Royal  Society,  A,  pp,  135,  1921. 

2  Monthly  Notices,  R.  A.  S.    Many  papers  beginning  83,  1923. 

3  Astrophysical  Journal,  55,  119  and  354,  1921. 

4  Eddington,  Internal  Constitution  of  the  Stars,  1926. 


Slit  set  at 


3968.6 


3968.2 


3967.8 


3966.4 


3965-0 


CALCIUM  FLOCCULI  PHOTOGRAPHED  BY  Fox  WITH  THE  4o-iNCH  YERKES  REFRACTOR, 

AUGUST  25,  1904 

The  slit  was  set  at  different  wave-lengths  corresponding  to  different  levels  above  the 
sun's  surface.    Order:  from  lowest  upwards. 


U.  S.  NAVAL  OBSERVATORY  ECLIPSE  STATION,  AUGUST  30,  1905 
The  40-foot  camera  is  placed  horizontally. 


LICK  OBSERVATORY  ECLIPSE  STATION,  JUNE  8,  1918 
The  40-foot  camera  is  pointed  directly  at  the  Sun, 


THK    IMPORTANCE    OF   IONIZATION 


301 


potential,  the  higher  must  be  the  temperature  to  sustain  a 
given  degree  of  ionization.  This  is  readily  seen  in  the  case  of 
helium  which  possesses  the  highest  known  ionization  poten- 
tial, of  24  volts ;  for  the  series  due  to  enhanced  helium  is  found 
only  in  stars  of  the  highest  temperature. 

Saha  calculates  tables  giving  the  percentage  of  ionization 
in  atmospheres  at  various  temperatures  and  pressures  ( meas- 
ured in  atmospheres).  The  following  tables  are  copied  from 
his  publication. 

PhKCKNTAGK    lONIZATION    OF    CALCIUM 


Pressure  fatmos  ) 

Temp. 

IO 

r 

10 

10  - 

IO 

10 

0 

4,000 

o 

0 

0 

3 

9 

26 

5,000 

o 

2 

6 

20 

55 

00 

6,000 

7 

8 

24 

64 

93 

99 

7,000 

7 

2} 

68 

91 

99 

IOO 

8,000 

16 

46 

84 

g8  5 

100 

IOO 

10,000 

46 

8^ 

g8  5 

IOO 

100 

IOO 

1  2,000 

76 

9<>  5 

100 

IOO 

IOO 

IOO 

14,000 

00 

IOO 

IOO 

IOO 

IOO 

IOO 

Pl'RCl-NTAr.h    lONI/ATION   OF    STRONTIUM 


Pressure  (atmos  ) 
Temp. 

IO 

I 

IO"1 

0 

_  



— 

4,000 

o 

O 

2 

5,000 

I 

3 

II 

6,000 

4 

13 

37 

7,000 

IO 

32 

7  •*> 

8,000 

2  2 

58 

91 

iorooo 

0 

go 

9<S  5 

I  2,000 

82 

97   5 

IOO 

14,000 

93 

IOO 

IOO 

to- 

10  4 

5 

73 

45 
g6 

78 

97 

IOO 

g6 

IOO 

IOO 

;g 

IOO 

IOO 

oo 

IOO 

IOO 

oo 

IOO 

JOO 

00 

IOO 

IOO 

Saha  applies  his  theory  to  explain  the  differences  between 
the  spectrum  of  the  sun  and  the  chromosphere,  and  Russell 
broadens  the  scope  of  the  theory  by  applying  it  to  show  the 
meaning  of  the  differences  in  intensities  of  lines  in  the  sun- 
spot  and  in  the  solar  spectrum.  The  results  furnish  a  com- 
plete triumph  for  the  Saha  theory. 


;j02  ECLIPSES    OF    THE    SUN 

Fortunately  for  the  theory,  the  flash  spectrum  gives  the 
height  in  kilometers  (or  miles),  that  the  various  vapors 
producing  different  spectral  lines  extend  above  the  level  of 
the  sun's  photosphere.  Within  the  past  few  years  there  has 
been  a  great  revision  of  ideas  regarding  the  pressures  found 
in  the  reversing  layer.  Where  formerly  it  was  regarded  that 
these  pressures  amounted  to  several  times  that  found  at 
sea  level  on  earth,  it  is  now  known  with  certainty  that  the 
chromospheric  pressures  are  even  less  than  a  thousandth  of 
an  atmosphere.  According  to  many  researches  in  recent 
years  by  a  number  of  competent  investigators,1  it  is  known 
that  the  solar  temperature  is  not  far  from  6000°  C.  abso- 
lute. Schwarzschild ~  has  shown  that  if  the  variation  in 
temperature  in  the  upper  atmosphere  is  caused  only  by  radia- 
tion, then  the  temperature  should  not  fall  below  6ooo°/2*, 
or  5000°,  approximately.  Hence  in  the  solar  atmosphere, 
where  the  temperatures  vary  between  5000°  and  6000°,  it  is 
easy  to  see  at  a  glance  from  Saha's  tables  the  percentage  of 
ionization.  For  instance,  in  the  chromosphere  at  elevations 
where  the  pressure  amounts  to  one  ten-thousandth  of  an  at- 
mosphere, ionization  for  calcium  is  ninety  per  cent  complete. 

Saha's  theory  is  in  complete  harmony  with  the  conclusions 
derived  from  the  discussion  of  the  flash  spectrum  and  fur- 
nishes an  adequate  explanation.  The  case  of  calcium  is 
most  interesting.  The  lines  H  and  K  are  enhanced  lines  and 
are  caused  by  the  ionized  atom,  while  the  g  line  at  4227  A 
takes  its  origin  from  the  neutral  atom.  In  the  neighborhood 
of  the  reversing  layer,  both  normal  and  ionized  atoms  will 
be  plentiful  and  the  presence  of  g  and  the  H  and  K  lines  are 
fully  explained.  At  great  heights  above  the  reversing  layer, 
however,  the  pressure  will  be  very  small  and,  as  a  result, 
ionization  will  be  nearly  complete.  Under  these  conditions 
the  neutral  atom  cannot  exist,  the  ionized  atom  exhibiting 
the  enhanced  lines  alone  being  found.  In  the  flash  spectrum, 
measures  indicate  that  the  H  and  K  lines  extend  upwards 
to  heights  of  14,000  kms,  but  the  g  line  only  to  5000  kms. 
The  presence  of  H  and  K  above  the  5000  km  level  shows 

1  Russell,  Dugan  and  Stewart,  Astronomy,  535,  1927. 
-  Gott.  Nachrichten,  41,  1906. 


THE    IMPORTANCE    OF    IONIZATION  303 

that  calcium  actually  exists  above  this  level,  and  we  must 
therefore  interpret  the  failure  of  the  atom  to  emit  the  g 
line  above  the  5000  km  level  to  be  due  to  the  fact  that  prac- 
tically all  of  the  atoms  are  ionized  and  there  are  few  normal 
atoms  left  to  produce  the  4227  line.  For  strontium  and 
barium,  which  also  exhibit  enhanced  lines,  their  ionization 
potentials  (5.7  and  5.1  volts)  are  lower  than  that  of  calcium 
(6.1  volts),  and  consequently  complete  ionization  in  the 
chromosphere  is  found  at  higher  pressures  or,  in  other  words, 
at  lower  elevations  above  the  photosphere.  The  strongest 
line  of  neutral  Sr  is  4607  A,  which  reaches  an  elevation  of 
only  400  km,  while  the  ionized  atom  Sr+  shows  the  two 
lines  at  4215  A  and  4077  A  which  are  found  at  elevations 
much  greater  than  that  of  the  neutral  atom,  in  fact,  at  6000 
kms,  but  this  level  is  much  less  than  the  14,000  km  height 
attained  by  the  H  and  K  lines  of  C0+- 

In  addition  to  Ca  and  Sr,  Ba  also  belongs  to  the  alkali 
earths  of  Group  II  of  the  periodic  table  of  the  chemical 
elements.  In  the  following  table,  there  are  given  the  details 
concerning  the  strongest  single  line  found  in  the  neutral  series 
of  each  of  the  three  elements,  and  also  the  strongest  doublet 
belonging  to  the  ionized  atom.  In  the  second  column  is  given 
the  wave-length  and  in  the  last  the  heights  in  kilometers 
measured  in  the  flash  spectrum.  In  the  four  other  columns 
are  given  the  intensities  in  sun,  chromosphere,  arc  and  spark, 
respectively.  The  intensities  for  the  laboratory  spectra  are 
taken  from  Exner  and  Haschek.  In  spite  of  its  high  atomic 
weight  ( 137.4),  Ba  is  a  conspicuous  element,  both  in  the  sun 
and  in  the  chromosphere,  on  account  of  the  enhanced  lines. 
No  neutral  lines  of  Ba  are  found  either  in  the  sun  or  in  the 
chromosphere. 

This  small  table  contains  some  of  the  conclusions  derived 
from  a  study  of  the  flash  spectrum  which,  on  account  of 
their  importance,  will  bear  being  repeated.  The  strongest 
line  in  the  Fraunhofer  spectrum  is  K  (Rowland  intensity 
1000)  and  this  line  is  the  strongest  in  the  flash  spectrum 
(intensity  200  on  a  different  scale).  The  enhanced  lines 
which  are  produced  by  the  ionized  atom  (designated  by  +) 
have  an  intensity  in  the  chromosphere  greater  than  in  the 


304 


ECLIPSES    OF    THE    SUN 


sun,  and  these  enhanced  lines  extend  to  greater  elevations 
above  the  photosphere  than  do  the  ordinary  or  unenhanced 
lines. 

In  the  flash  spectrum  the  elements  arranged  according  to 
the  intensity  of  the  strongest  lines  give  the  following  se- 
quence: Ca,  H,  He,  Tiy  Mg,  Sr,  Sc,  Cr,  Ba,  Fe,  A I  and  Y. 
This  order  differs  materially  from  a  similar  table  for  the 

COMPARISONS  OF  THE  STRONGEST  LINKS  OF  THE  NKUTR\L  AND  IONI/ED  ATOMS 


Klement 

Wave-Length 

Sun 

Chromo- 
sphere 

Arc 

Spirk 

Height 
in  km 

Ca 

4^27  (g) 

20 

40 

IOOO 

TOO 

S,ooo 

CVi-{- 

303?  (K> 

IOOO 

2OO 

=500 

IOOO 

I4,OOO 

Ctf-h 

3Q6g  (H) 

700 

'75 

300 

500 

T4,OOO 

.Sr 

4607 

I 

2 

1OOO 

50 

400 

.SV-h 

40/7 

8 

80 

IOOO 

IOOO 

(>,ooo 

Sr+ 

4215 

5 

60 

500 

500 

(),OOO 

Bu, 

5535 

— 

_ 

loo 

3" 

— 

Ba-{- 

4554 

8 

5° 

rooo 

IOOO 

2,000 

Ba  + 

4Q34 

7 

^5 

IOO 

300 

J,2OO 

ordinary  solar  spectrum.  In  lists  arranged  according  to  the 
number  of  lines,  Fe  stands  first  in  each  case.  Ni  and  Co 
are  second  and  sixth  in  the  solar  spectrum,  but  become 
sixth  and  eighth,  respectively,  in  the  flash  spectrum.  V  is 
the  eighth  element  in  the  Fraunhofer  spectrum  but  fourth 
in  the  chromosphere.  Cb,  Mo  and  Pd  appear  in  the  first 
list  but  not  at  all  in  the  second,  while  in  the  chromosphere 
is  found  He  which  is  absent  in  the  ordinary  spectrum.  The 
rare  earths  are  represented  relatively  by  more  lines  in  the 
flash  than  in  the  ordinary  spectrum. 

Interesting  results  are  obtained  when  the  total  number 
of  lines  identified  in  the  flash  spectrum  are  arranged  accord- 
ing to  the  periodic  tables  of  the  elements.  This  is  given  in 
the  table  on  page  306. 

For  each  element  in  the  table  there  is  given:  in  the  first 
line  the  atomic  number  and  the  chemical  symbol,  in  the 
second  line  is  found  the  atomic  weight,  and  in  the  third  line 
in  italics  the  total  number  of  lines  in  the  chromosphere. 


THE    IMPORTANCE    OK    IONIZATION  305 

The  mark  (?)  is  found  with  the  elements  Cb,  Mo,  Ag  and 
Cd  to  signify  that  there  are  no  strong  lines  in  the  chromo- 
sphere resulting  from  these  elements,  but  they  may  possibly 
be  represented  by  weak  lines  in  combination  with  stronger 
lines  of  other  elements.  Eight  of  the  rare  earths  are  repre- 
sented in  the  flash  spectrum  by  a  total  of  1 78  lines,  Ce,  La 
and  Nd being  responsible  for  57, 47  and  31  lines,  respectively. 
All  of  the  rare-earth  lines  without  exception  are  enhanced 
lines.  The  element  Ca  occupies  an  interesting  place  in  the 
table.  To  the  right  of  Ca  in  the  table  are  found  Sc,  Ti,  V,  Cr, 
Mn,  Fe,  Co  and  Ni,  all  represented  by  numerous  strong  lines 
both  in  chromosphere  and  in  Fraunhofer  spectrum.  Verti- 
cally above  Ca  is  Mg,  and  below  it  are  Sr  and  Ba,  all  impor- 
tant elements  in  solar  investigations.  In  column  I,  of  the 
alkali  metals,  the  only  element  represented  in  the  chromo- 
sphere is  Na,  where  it  is  found  with  less  prominence  than  in 
the  Fraunhofer  spectrum.  He  is  the  only  element  in  column 
O,  the  inert  gases,  that  manifests  itself  in  the  flash  spectrum. 

As  mentioned  above,  the  chromospheric  and  solar  spectra 
agree  exactly  as  to  wave-lengths,  but  differ  very  greatly  in 
the  relative  intensities  of  the  lines.  These  differences  of 
intensity  are  accentuated  in  the  case  of  the  "  enhanced  " 
lines,  or  those  which  are  more  intense  in  the  spectrum  of 
the  spark  than  in  the  arc. 

A  comparison  of  the  intensities  of  the  lines  in  the  Fraun- 
hofer, chromospheric,  arc  and  spark  spectra  forces  one  to 
the  conclusion  that  while  the  Fraunhofer  spectrum  corre- 
sponds to  the  arc  spectrum,  the  spectrum  of  the  chromo- 
sphere more  closely  resembles  that  of  the  spark  spectrum. 
The  sun  thus  exhibits  three  distinct  spectra  under  different 
conditions:  the  chromospheric  spectrum,  the  Fraunhofer 
spectrum  and  the  sun-spot  spectrum.  These  three  closely 
resemble  the  spectra  of  the  stars  y  Cygni,  Capella  and  Arc- 
turus,  respectively;  y  Cygni  representing  an  "  earlier  "  and 
Arcturus  a  "  later  "  type  of  spectrum  than  that  of  the  sun. 
Hence,  the  Harvard  classification  of  the  three  spectra  are: 
chromosphere  =  Fo,  sun  =  Go  and  sun-spot  spectrum  =  Ko. 

A  comparison  of  the  spectra  of  the  sun  and  the  chromo- 
sphere reveals  the  following  interesting  facts:  first,  the 


306 


ECLIPSES    OF    THE    SUN 


g 


o 

</5  C 

H  ** 

g  -s 

1  -S 

r^  rt 

f  T    ]  |Jj 


a    c: 

H 


CO         ^ 

•*£        *-" 

r    ,  ^j 


PH        3 

4; 


c 
o 


10  O 


<N    in  t-s 


CQ 


0§ 

cc  "2 


L.   O 


: 


O    0 

-t  a 


O 

c 


s:  x 

^   -t 
c  ^ 


"3,2 


n 

9       ^ 


s2.8 


I-  O 

-t  >-l 


'0VJ 


THK    IMPORTANCE    OF    IONIZATION  307 

values  of  Rowland,  giving  the  intensities  of  the  lines  of  the 
ordinary  solar  spectrum,  are  comparable  with  arc  intensities, 
while  those  of  the  chromosphere  approximate  more  closely 
to  the  intensities  of  the  spark;  and  second,  the  enhanced 
lines  of  any  element  are  in  every  case  stronger  in  the  chromo- 
sphere than  the  lines  of  the  neutral  atom,  and  the  heights 
attained  are  much  greater.  The  results  are  a  complete  veri- 
fication of  Saha's  theory  that  enhanced  lines  are  caused  by 
ionization  which  becomes  more  and  more  complete  in  the 
sun's  upper  atmosphere  where  the  great  altitudes  make  the 
pressures  very  small. 

Not  only  is  Saha's  theory  able  to  explain  the  facts  regard- 
ing the  enhanced  lines  of  the  ionized  atom,  but  it  makes 
clearer  the  details  concerning  the  lines  of  the  neutral,  or 
un-ionized  atom.  Take,  for  example,  the  D-lines  of  sodium, 
so  well  known  in  the  Fraunhofer  spectrum.  At  pressures 
below  one-thousandth  of  an  atmosphere,  Na  with  an  ioniza- 
tion potential  of  5.1  volts,  is  completely  ionized.  The  D- 
lines  belong  to  the  principal  series  of  the  normal  atom,  and 
accordingly,  they  have  no  connection  with,  and  are  not  pro- 
duced by,  the  ionized  atom.  The  normal  atoms  forming  the 
D-lines  therefore  cannot  exist  when  the  pressure  in  the 
chromosphere  is  reduced  to  the  thousandth  of  an  atmos- 
phere. It  is  quite  in  keeping  with  theory  to  find  the  flash 
spectrum  photographs  furnishing  the  information  that  the 
D-lines  reach  the  comparatively  small  heights  of  only 
1500  kms  above  the  photosphere  (H  and  K  are  found  at 
14,000  kms).  The  contrast  in  behavior  in  passing  from 
the  Fraunhofer  to  the  flash  spectrum  for  the  D-lines  of 
sodium  on  the  one  hand,  and  Dt  of  helium  on  the  other, 
is  very  marked.  The  sodium  lines  are  weakened  in  the 
flash  while  the  helium  line  is  enormously  strengthened, 
being  entirely  lacking  in  the  ordinary  solar  spectrum.  Fur- 
thermore, in  view  of  the  great  prominence  of  the  D-lines  in 
the  solar  spectrum,  it  has  always  been  a  matter  of  the  great- 
est surprise  that  the  element  potassium,  so  similar  in  its  prop- 
erties to  sodium,  is  not  found  represented  by  strong  lines  in 
the  sun.  The  explanation  is  a  very  simple  one.  The  lines 
of  the  neutral  atom  of  potassium,  corresponding  in  its  series 


308  ECLIPSES    OF    THE    SUNr 

to  the  D-lines  of  sodium,  are  found  in  the  deep  red  part  of 
the  spectrum  at  wave-lengths  7664  A  and  7699  A,  and  con- 
sequently they  are  not  in  the  visible  spectrum.  Like  the 
D-lines,  both  lines  of  this  pair  are  strengthened  in  sun-spots. 
The  only  lines  due  to  potassium  found  in  the  visible  solar 
spectrum  are  very  weak  lines  at  4044  A  and  4047  A,  of 
Rowland  intensities  o  and  oo,  respectively.  Russell  finds 
both  these  lines  strengthened  in  sun-spots.  No  enhanced 
lines  are  known  for  Na  or  for  K,  and  consequently  neither 
element  is  conspicuous  in  the  flash  spectrum. 

The  temperature  of  the  photosphere  is  approximately 
6000°,  while  that  of  sun-spots  is  lower  and  probably  some- 
where near  4700°.'  The  pressures  found  in  sun-spots  can 
differ  but  little  from  those  in  the  lowest  depths  of  the  re- 
versing layer.  On  account  of  the  lower  temperatures  in  the 
spots,  however,  ionization  is  less  complete  according  to 
Saha's  theory.  As  a  result,  the  lines  of  the  neutral  atom, 
the  so-called  "  low  temperature  "  lines,  are  strengthened  in 
sun-spots,  while  on  the  other  hand,  and  also  as  a  direct  con- 
sequence of  Saha's  theory,  the  enhanced,  or  u  high-tempera- 
ture "  lines  are  weakened  in  the  spectrum  of  sun-spots. 
Since  the  variations  in  pressure  in  the  neighborhood  of  the 
sun  are  much  greater  than  the  variations  in  temperature, 
it  would  have  been  more  fortunate  if  the  enhanced  lines 
had  been  referred  to  as  "low-pressure"  rather  than  as 
"  high-temperature  "  lines. 

In  the  light  of  Saha's  theory,  Russell  has  investigated  the 
sun-spot  lines.  His  conclusions  for  the  alkali  metals  (loc. 
cit.,  p.  129)  are  here  briefly  given.  Sodium  is  represented 
in  the  sun  by  the  principal,  diffuse  and  sharp  lines  of  the 
neutral  series,  and  all  of  its  lines  are  much  strengthened  in 
the  spot  spectrum.  Potassium  is  represented  by  the  principal 
series  only  and  its  lines  are  also  strengthened  in  sun-spots. 
Lithium  is  found  in  the  spot  spectrum  only  at  wave-length 
6707  A.  Rubidium,  hitherto  unknown  in  the  sun,  was  dis- 
covered to  be  present  by  both  members  of  the  strongest 
pair  of  the  principal  series,  the  wave-lengths  being  7800  A 
and  7947  A. 

1  Miss  Moore,  Astro  physical  Journal,  75,  222,  1932. 


THE    IMPORTANCE    OF    IONIZATION  309 

We  are  now  in  a  position  to  explain  some  of  the  peculiari- 
ties regarding  the  appearance  of  lines  in  the  spectrum  of  the 
sun  and  chromosphere  and  the  heights  found  in  the  flash 
spectrum.  The  peculiarities  noted  (Chapter  XVI)  are  as 
follows:  The  H  and  K  lines  of  calcium  of  atomic  weight  40 
are  stronger  in  sun  and  chromosphere  and  reach  greater 
heights  than  hydrogen,  the  lightest  gas  in  the  sun.  In 
the  chromosphere  the  whole  Balmer  series  for  hydrogen  is 
found,  while  only  the  first  four  members  are  seen  in  the 
Fraunhofer  spectrum.  No  helium  lines  are  found  in  the 
ordinary  solar  spectrum,  but  they  are  of  great  strength  in 
the  chromosphere.  The  elements,  other  than  H  and  He,  ar- 
ranged according  to  the  periodic  table  (see  page  306)  have 
remarkable  progressions  in  the  number  and  intensities  of 
the  lines  involved.  Group  II,  the  alkali  earths,  represent  the 
strong  lines  in  the  chromosphere,  the  strongest  lines  of 
all  belonging  to  Ca.  Group  I,  the  alkali  metals,  have  few 
strong  lines  in  sun  or  chromosphere  other  than  the  D-lines 
of  Na.  None  of  the  lines  of  Group  O  originating  from  the 
inert  gases  Nc,  A,  Kr  and  Xe  are  found  in  sun  or  chromo- 
sphere. In  Group  III,  strong  lines  are  found  for  Aly  Sc,  Y, 
and  the  rare  earths,  but  the  strength  of  lines  is  not  as  great 
as  reached  by  the  corresponding  elements  in  Group  II.  In 
Group  IV  intensities  are  still  less.  The  only  element  in 
Group  V,  found  with  certainty  in  the  chromosphere,  is  va- 
nadium, and  in  Groups  VI  and  VII,  O  and  Cr,  and  Mn,  re- 
spectively, and  in  Group  VIII,  the  three  metals  Fey  Co, 
and  Ni. 

It  is  easy  to  see  why  the  metals  of  Group  I,  the  alkalis, 
are  represented  by  such  feeble  lines  in  the  chromospheric 
spectrum.  For  reasons  already  stated,  the  enhanced  spectra 
of  the  alkali  metals  resemble  the  spectra  of  the  neutral  atoms 
in  the  preceding  group  in  the  periodic  table,  the  inert  gases; 
and  consequently,  such  spectra  are  very  difficult  to  produce 
on  account  of  the  outer  electrons  forming  part  of  a  very 
stable  ring  or  shell.  As  a  matter  of  fact,  no  enhanced  lines 
are  found  for  any  of  the  alkali  metals  in  the  visible  portion 
of  the  spectrum.  It  is  apparent,  therefore,  why  the  alkali 
metals  cannot  be  prominently  represented  in  the  chromo- 


310  ECLIPSES    OF    THE    SUN 

sphere  since  the  flash  spectrum  is  essentially  an  enhanced 
spectrum. 

Quite  different  is  the  situation  regarding  the  elements  of 
Group  II,  the  alkali  earths,  which  are  specially  important 
in  the  chromospheric  spectrum,  for  the  reason  that  the 
strongest  lines  of  their  spectra  are  enhanced  lines,  and  the 
principal  members  ( is —  2p]  of  the  series  lie  in  the  familiar 
portion  of  the  spectrum.  This  is  true  for  the  elements  with 
the  exception  of  Mg,  the  strongest  lines  of  which  are  found 
in  the  extreme  ultra-violet  at  2795  A  and  2802  A,  in  a  region 
in  fact  where  no  light  can  reach  the  earth's  surface  from 
the  sun  on  account  of  the  absorption  of  light  in  the  earth's 
atmosphere. 

The  great  strength  of  the  H  and  K  lines  of  calcium  both 
in  the  sun  and  in  the  chromosphere  and  the  great  heights  to 
which  these  lines  extend  in  the  flash  spectrum  are  now 
completely  explained  as  the  result  of  Saha's  theory.  In 
spite  of  the  great  difference  in  the  atomic  weights  of  the 
two  gases,  calcium  and  hydrogen,  the  atomic  weight  of  the 
former  being  forty  times  the  latter,  the  spectrum  lines  of 
calcium  are  seen  to  reach  greater  heights  than  are  attained 
by  hydrogen.  The  reasons  for  this  curious  circumstance 
are  very  simple.  H  and  K  are  lines  due  to  the  ionized  atom, 
and  in  virtue  of  the  great  elevations  the  ionization  is  greatly 
increased.  The  lines  H  and  K  are  the  chief  lines  belonging 
to  the  principal  series,  and  in  fact  are  the  only  lines  of  this 
series  in  the  chromosphere.  Only  five  others  lines  of  Ca+ 
are  found  in  the  chromosphere,  they  are  all  of  subordinate 
series  and  they  have  much  smaller  intensities  than  H  and 
K.  The  hydrogen  lines  in  the  visible  spectrum,  on  the  con- 
trary, belong  to  a  subordinate  series  and  not  to  the  princi- 
pal one. 

In  the  above,  attention  has  been  called  to  the  great  differ- 
ences in  the  spectra  observed  in  sun,  chromosphere  and  sun- 
spots.  To  find  an  explanation  of  the  observed  facts,  it  is 
necessary  to  look  more  closely  into  the  newer  developments 
regarding  the  structure  of  the  atom.  In  Chapter  XIV  it  was 
shown  that  many  atomic  characteristics  are  explained  on  the 
assumption  that  the  electrons  forming  the  more  complex 


THE    IMPORTANCE    OF   IONIZATION  311 

atoms  are  arranged  in  successive  shells.  The  inert  gases 
helium,  etc.  in  column  O  of  the  periodic  table,  have  atomic 
numbers  of  2,  10,  18,  36,  54  and  86,  and  hence  it  has  been 
assumed  that  the  concentric  shells  contain  2,  8,  8,  18,  18  and 
32  electrons,  respectively. 

Lithium  of  atomic  number  3,  and  sodium  of  atomic  num- 
ber n,  are  elements  which  have  one  external  electron  out- 
side of  a  complete  shell.  The  process  of  removing  an  outer 
electron  from  an  atom  is  called  ionization.  This  may  be 
accomplished  by  various  electrical  processes,  some  of  which 
permit  the  measurement  of  the  energy  required  in  volts.  As 
each  electron  carries  a  unit  charge  of  negative  electricity, 
an  ionized  atom,  called  an  "  ion,"  has  a  net  positive  charge 
of  electricity.  As  we  have  seen,  the  spectrum  of  the  ionized 
atom  is  radically  different  from  that  of  the  neutral  atom. 
For  reasons  stated  in  the  foregoing,  the  spectral  lines  be- 
longing to  the  ionized  atoms  are  called  "  spark  "  or  "  en- 
hanced "  lines. 

Not  only  can  an  atom  be  removed  from  an  outer  shell 
through  ionization,  but  a  given  atom  or  ion  can  be  excited. 
That  is,  the  position  of  one  or  more  of  the  electrons  can  be 
changed  within  the  atom  itself.  To  change  from  the  normal 
state  to  an  excited  state  requires  energy.  If  this  energy 
is  absorbed  by  the  atom,  the  spectrum  refers  to  the  dark  line 
or  absorption  spectrum.  When  the  change  takes  place  in 
the  reverse  direction,  from  excited  state  to  normal  state,  the 
same  amount  of  energy  is  emitted  by  the  atom  and  the 
spectrum  is  a  bright-line  or  emission  spectrum.  As  already 
stated  in  a  previous  chapter,  the  transference  of  energy 
from  one  state  to  another  is  always  in  definite  amounts,  or 
quanta,  of  energy. 

It  has  been  found  that  the  simple  element  hydrogen  can 
exist  in  more  than  twenty  excited  states.  More  than  one 
hundred  different  energy  levels  have  already  been  discovered 
by  spectroscopic  means  for  the  more  complicated  atom 
iron.  The  transference  of  energy  from  the  excited  to  the 
normal  state  has  been  found  to  take  place  in  the  brief  inter- 
val of  time  of  approximately  one-millionth  of  a  second. 

Planck's  quantum  theory  is  justly  regarded  as  one  of  the 


312  ECLIPSES    OF    THE    SUN 

most  important  triumphs  of  the  twentieth  century.  By 
means  of  it,  we  know  that  the  frequency  of  the  radiation 
emitted  by  the  atom,  and  hence  the  wave-length,  is  exactly 
proportioned  to  the  amount  of  energy  radiated. 

In  the  case  of  hydrogen,  we  have  seen  that  the  wave- 
lengths of  the  35  lines  which  appear  in  the  chromosphere  can 
be  represented  by  the  difference  between  two  spectroscopic 
terms.  With  ordinary  hydrogen  and  enhanced  helium  there 
is  only  one  external  electron.  When  there  are  two  or  more 
electrons,  the  mathematical  computations  become  very  com- 
plicated and  as  a  result  the  exact  calculations  and  predictions 
of  spectral  lines  are  impossible.  However,  there  are  sound 
reasons  for  believing  that  all  electrons  in  the  outer  shells  of 
atoms,  no  matter  how  complicated  these  atoms  may  be, 
obey  rules,  called  quantum  conditions,  that  are  quite  similar 
to  those  underlying  the  simple  structures  of  hydrogen  and 
enhanced  helium.  The  changes  in  the  outer  electrons  give 
rise  to  radiations  which  produce  spectral  lines.  The  energy 
of  the  electron  depends  on  the  relation  of  the  size,  shape  and 
inclination  of  the  orbits  to  each  other,  and  hence  with  the 
more  complicated  atoms,  the  number  of  different  energy 
states,  or  spectroscopic  terms,  is  much  greater  than  in  the 
simple  case  of  hydrogen. 

This  is  not  the  place  to  go  into  the  technical  details  in- 
volved in  the  transference  of  energy  from  one  spectroscopic 
state  to  another.  For  the  more  complicated  atoms,  it  is  im- 
possible to  represent  the  orbits  as  was  done  so  simply  for 
hydrogen  in  Figure  4.  However,  by  means  of  a  diagram 
suggested  by  Bohr  and  Grotrian,  the  different  energy  states 
of  an  atom  can  be  represented  quite  readily.  Each  state  is 
represented  by  a  dot.  The  distance  below  a  heavy  horizon- 
tal line  is  proportional  to  the  amount  of  energy  ( measured 
in  volts)  which  must  be  communicated  to  the  external  elec- 
tron to  pull  it  entirely  away  from  the  atom  and  thence  to 
ionize  the  atom.  The  lowest  dots  in  the  diagram  represent 
those  of  least  energy  while  the  heavy  horizontal  line  repre- 
sents the  ionized  condition. 

The  rules  of  the  game  '  laid  down  for  the  electrons  to  fol- 

1  See  Russell,  Dugan  and  Stewart,  Astronomy,  548,  1927. 


THE    IMPORTANCE    OF   IONIZATION  313 

low  are  not  complicated.  In  the  diagram,  states  correspond- 
ing to  orbits  of  the  same  angular  momentum  plotted  by 
dots  in  the  same  column  thus  s,  py  dy  f,  gy  h,  etc.,  represent 
conditions  of  increasing  angular  momentum.  When  there  is 
a  transference  of  energy  from  one  state  to  another,  two  of 
the  dots  are  joined  by  a  line.  An  atom  in  the  is  state  (the 
lowest  energy  state  in  the  s  column)  can  change  to  any  of  the 
p  states  but  not  to  the  others;  or  an  atom  in  the  ip  state  can 
change  to  any  s  or  d  state,  and  so  forth. 

In  the  sodium  spectrum,  for  instance,  when  a  small  amount 
of  heat  is  applied,  as  in  the  Bunsen  burner,  the  only  lines 
emitted  are  those  confined  to  the  two  lowest  levels  s  and  p. 
The  lines  emitted  correspond  to  transitions  from  the  p  states 
to  the  lowest  s  state.  If  white  light  is  passed  through  sodium 
vapor  these  lines  are  absorbed.  This  series  of  lines  is  desig- 
nated by  is  —  mp.  The  level  is  is  the  normal  state  of 
the  sodium  atom  in  consequence  of  the  lowest  amount  of 
energy.  If  the  atom  is  excited  and  is  in  the  ip  state,  it  can 
return  to  the  normal  state  only  by  the  radiation  of  a  quan- 
tum of  energy  corresponding  to  the  transition  15 — ip, 
which  causes  the  appearance  of  the  familiar  D  lines  in  the 
spectrum. 

The  transitions  is  —  mp  are  called  the  principal  series. 
If  the  temperature  is  raised  above  that  of  the  Bunsen  burner, 
by  the  application  of  electricity,  the  atom  can  exist  in  higher 
energy  states,  such  as  2^  and  2d.  With  increase  of  tempera- 
ture, the  fraction  of  atoms  in  the  ip  state,  though  quite  small, 
increases  and  hence  the  intensity  of  the  lines  become  greater. 
At  still  higher  temperatures,  the  fractions  of  the  atoms  in 
the  2$  and  2d  states  continually  increase,  with  the  consequent 
emission  of  lines  represented  by  the  transitions  ip  —  2s 
and  ip —  2d,  etc. 

If  sodium  is  placed  in  the  electric  arc,  then  it  is  found  that 
all  the  lines  of  the  series  is  —  mp  are  sharp  while  the  lines 
ip  —  md  are  more  fuzzy  or  diffuse.  Hence  the  significance 
of  the  letters  s  and  d. 

The  amount  of  energy  required  to  raise  an  atom  from  the 
normal  state  to  any  given  excited  state  is  called  the  excitation 
potential  which  is  expressed  in  volts.  A  simple  formula  con- 


ECLIPSES    OF    THE    SUN 

nects  the  energy  7,  in  volts,  with  the  wave-length  A,  in  ang- 
stroms as  follows. 

V  _i^45 
A 

The  p  levels  of  Na,  and  also  of  all  the  alkali  metals  are 
double,  and  hence  the  transitions  cause  double  lines  in  all 
the  elements  in  column  I  of  the  periodic  table  just  as  with 
the  familiar  D  lines  of  sodium. 

When  there  is  more  than  one  valence  electron,  the  spectra 
are  much  more  complex  than  that  of  sodium.  There  are 
then  many  different  energy  states  and  these  may  be  single, 
double,  triple  and  so  forth  up  to  eight-fold.  The  transitions 
from  one  energy  level  to  another  level  give  rise  to  numbers 
of  lines  known  as  multiplet  groups.  These  may  contain  as 
many  as  fifteen  members  of  varying  intensities,  some  lines 
strong  and  some  weak. 

All  of  these  important  spectroscopic  developments  have 
taken  place  in  the  past  ten  years.  The  chief  workers  in 
bringing  order  out  of  chaos,  in  the  grouping  of  spectral  lines 
into  multiplets  and  deriving  the  excitation  potentials  have 
been  A.  Fowler,  Russell,  Saunders,  Meggers  and  Miss  Moore. 
Their  investigations  have  been  greatly  helped  through  accu- 
rate wave-lengths  furnished  by  King  of  Mt.  Wilson  and 
Meggers  of  the  Bureau  of  Standards. 

The  method  of  finding  series  relationships  is  somewhat 
similar  to  that  used  for  the  simple  element  hydrogen.  In- 
stead of  working  with  the  observed  wave-lengths  it  is  found 
to  be  easier  to  work  with  frequencies,  or  the  reciprocal  of 
wave-lengths.  Certain  rules  have  been  evolved  to  which  the 
frequencies  are  subjected.  Then  in  a  manner  somewhat  simi- 
lar to  that  of  working  out  a  Chinese  puzzle,  relationships 
are  gradually  discovered  between  the  lines  of  a  certain 
element,  and  finally  these  lines  can  be  grouped  into 
multiplets. 

Not  only  are  the  wave-lengths,  or  frequencies  of  the  spec- 
tral lines  found  to  obey  the  quantum  conditions,  but  the 
intensities  are  also  subject  to  laws.  Since  1922  the  prog- 
ress in  this  work  has  been  very  rapid  until  now  all  of  the  im- 


THE    IMPORTANCE    OF    IONIZATION  315 

portant  lines  in  the  spectra  of  most  of  the  elements  have  been 
interpreted. 

In  unravelling  the  puzzle,  King's  temperature  classes  have 
been  of  the  greatest  use.  Lines  of  Class  I  appear  at  the  low- 
est temperatures,  that  of  the  Bunsen  flame.  As  the  tem- 
perature is  raised,  first  through  the  electric  furnace  and  then 
at  the  higher  and  higher  temperatures  of  the  arc  and  spark, 
lines  of  Classes  II,  III,  IV  and  V  successively  appear  and  the 
lines  of  the  earlier  classes  become  stronger  and  stronger. 
Class  V  lines  appear  in  the  arc  only.  Conversely,  at  lower 
and  lower  temperatures,  all  of  the  lines  become  progressively 
weaker  and  lines  disappear  in  groups.  The  last  lines  to  dis- 
appear, those  that  are  visible  even  at  the  lowest  tempera- 
tures, are  called  ultimate  lines.  These  correspond  to  transi- 
tions to  or  from  the  lowest  level.  Lines  from  higher  levels 
form  subordinate  series. 

With  increase  of  temperature  the  energy  to  which  the 
atoms  are  subjected  increases.  The  rate  at  which  atoms  are 
ionized  increases  with  the  temperature.  If  the  electrons 
liberated  from  the  atom  by  ionization  could  be  kept  from 
combining  with  some  other  atom,  then  the  process  would  go 
on  until  there  were  no  more  neutral  atoms  left.  However, 
the  liberated  electron  soon  meets  another  ion  and  combines, 
again  forming  a  neutral  atom.  Evidently  the  relative  num- 
bers of  neutral  and  ionized  atoms  depend  on  conditions. 
High  temperature  increases  the  rate  at  which  atoms  are 
ionized  while  on  the  other  hand  high  density,  where  atoms  are 
crowded  together,  increases  the  rate  of  recombination  into 
neutral  atoms.  Hence  it  is  evident  that  at  a  constant  tem- 
perature, ionization  increases  in  amount  with  decrease  in 
density  or  pressure.  At  the  same  time  it  is  evident  that  at 
the  same  temperature  and  pressure  those  elements  can  give 
up  an  external  electron  most  freely  when  the  energy  required 
for  this  process,  the  ionization  potential,  is  the  least.  Or  in 
other  words,  the  greater  the  ionization  potential,  the  less  the 
degree  of  ionization. 

As  already  stated,  from  a  knowledge  of  heights  to  which 
vapors  ascend  in  the  chromosphere,  Saha  was  able  to  show 
that  close  to  the  photosphere  where  the  temperatures 


316  ECLIPSES    OF    THE    SUN 

changed  but  little,  increase  of  height  meant  decrease  of 
density  and  pressure  and  a  greater  facility  whereby  atoms 
became  ionized.  Hence  with  the  lines  of  the  spectra  grouped 
into  multiplets  for  which  excitation  potentials  are  known,  we 
are  enabled  to  explain  quite  simply  most  of  the  problems 
connected  with  the  chromosphere. 

Let  us  take  as  an  example,  the  interesting  element  radium 
in  order  to  see  whether  or  not  it  is  in  the  sun.  Twenty  years 
ago  there  was  much  discussion  over  the  question,  Dyson 
having  found  flash  spectrum  lines  due  to  radium,  while 
Evershed  and  Mitchell  took  the  opposite  stand.  The  strong- 
est lines  of  the  principal  series  of  enhanced  radium  are  at 
3814  A  and  4682  A,  while  three  lines  belonging  to  the  diffuse 
series  are  at  wave-lengths  3649  A,  4340  A  and  4436  A. 
These  are  the  strongest  lines  of  Ra+,  and  if  radium  is  in  the 
chromosphere,  we  should  unquestionably  expect  it  to  display 
its  presence  by  these  wave-lengths.  As  shown  in  Popular 
Astronomy,  21,  321,  1913,  each  of  these  five  lines  is  already 
satisfactorily  identified  by  coincidences  with  lines  in  Row- 
land's Tables,  without  invoking  radium  as  a  source.  If 
therefore,  we  are  to  prove  that  radium  is  in  the  chromosphere, 
it  will  be  possible  only  as  the  result  of  flash  spectrum  photo- 
graphs with  much  greater  dispersion  than  those  taken  up  to 
the  present. 

The  case  of  the  element  Mg  is  specially  interesting  for 
the  reason  that  it  is  one  of  the  alkali  metals  and  is  found  in 
Column  I  of  the  periodic  table  immediately  above  the  ele- 
ment Ca.  The  strongest  lines  of  the  solar  and  chromo- 
spheric  spectra  are  the  H  and  K  lines  of  enhanced  calcium. 
Unquestionably  Mg  is  very  abundant  in  the  sun,  but  why 
then  are  there  no  very  intense  lines  of  ionized  M g  found  in 
sun  and  chromosphere?  The  reason  is  not  that  the  ioniza- 
tion  potential  is  too  high,  for  the  values  of  Ca  and  Mg 
differ  little,  being  6.1  and  7.6  volts,  respectively.  In  the 
chromosphere  appear  the  three  lines  of  the  well-known 
6-group  in  the  green,  there  is  a  triplet  in  the  violet  at  3829, 
3832  and  3838  A,  all  ascending  to  heights  of  6000  or  7000 
km.  Farther  to  the  violet  is  another  triplet  at  3329,  3332 
3336  A.  Each  of  the  three  triplets  has  an  excitation 


THE    IMPORTANCE    OF   IONI/ATION  317 

potential  of  2.70  volts.  In  addition  there  are  many  singlet 
M g  lines  in  the  chromosphere  of  the  higher  excitation  poten- 
tial of  4.33  volts.  These  lines  of  Mg  are  all  of  them  much 
more  prominent  than  the  corresponding  lines  in  the  spectrum 
of  Ca.  Hence  it  appears  certain  that  Mg  is  a  more  promi- 
nent, and  thus  a  more  abundant  element  in  the  sun  than  Ca. 
But  where  are  the  ultimate  lines  of  enhanced  Mg  corre- 
sponding to  the  H  and  K  lines  of  enhanced  Ca?  The  answer 
has  already  been  given.  They  are  far  in  the  ultra-violet,  at 
wave-lengths  2795  and  2802,  and  on  account  of  the  absorp- 
tion in  our  terrestrial  atmosphere  they  cannot  be  photo- 
graphed either  in  the  sun  or  in  the  chromosphere.  The  only 
line  of  Mg-\-  of  any  importance  in  either  solar  or  stellar 
spectra  is  the  line  at  wave-length  4481  A.  In  the  flash  spec- 
trum this  line  exists  but  it  is  comparatively  weak  and  is  de- 
tected only  to  a  height  of  400  km.  The  reason  is  now  clear. 
This  line  has  a  high  excitation  potential  of  8.82  volts,  it 
does  not  appear  at  all  in  the  arc  spectrum.  Hence  its  pres- 
ence only  in  the  spectra  of  the  hotter  stars  is  readily  ex- 
plained. 

Saha's  theory  thus  interprets  in  a  beautifully  clear  man- 
ner the  systematic  differences  between  the  flash  spectrum, 
the  solar  spectrum  and  the  sun-spot  spectrum.  It  goes 
much  further,  however,  and  furnishes  the  causes  of  the 
progression  in  type  of  the  stars  from  the  red  stars  of  class  M 
to  the  early  types  of  B  and  O.  Lockyer  was  the  first  to  call 
attention  to  the  change  in  the  appearance  of  the  lines  H  and 
K,  very  faint  or  even  missing  in  late  M  stars,  with  a  maxi- 
mum intensity  in  the  solar,  or  Go  stars,  and  becoming  faint 
again  in  early  B  stars  and  disappearing  in  certain  O  stars. 
Lockyer 's  interpretation,  one  of  temperature  only,  was  un- 
satisfactory. The  hydrogen  lines  have  their  maximum  at 
type  Ao  and  are  less  intense  in  both  the  earlier  and  later 
types.  The  lines  of  neutral  helium  appear  only  in  the  stars 
of  very  early  type,  while  the  4686  line  and  the  Pickering 
series  due  to  enhanced  helium  are  found  only  in  still  earlier 
types.  The  conditions  of  appearance  and  disappearance  of 
spectral  lines  due  to  ionization  are  calculable,  and  it  has 
thus  been  possible  to  assign  temperatures  to  stars  of  dif- 


,'318  ECLIPSES    OF    THE    SUN 

ferent  types  which  are  in  substantial  agreement  with  those 
derived  from  other  lines  of  research.  All  of  the  difficulties 
have  not  been  entirely  cleared  away,  but  there  has  been  a 
great  step  forward. 

The  theory  of  ionization  has  shown  the  essential  unity  of 
astronomy,  proving  not  only  that  the  sun  is  a  typical  star 
but  also  that  a  study  of  the  stars  can  shed  much  information 
on  solar  questions.  The  great  problem  of  astronomy,  the 
evolution  of  the  stars  and  the  structure  of  the  universe,  can 
find  their  complete  solution  only  through  an  intimate  study 
of  the  ultimate  constitution  of  matter.  The  size  of  an  atom 
can  be  deduced  from  observations  of  the  gigantic  stars.  The 
structure  of  the  atom  and  the  theory  of  ionization  are  un- 
questionably among  the  most  important  problems  of  present- 
day  physical  science. 


CHAPTER  XVIII 

HEIGHTS  AND  RELATED  PROBLEMS 

THE  conclusions  drawn  from  eclipse  spectra  that 
strong  lines  originate  at  higher  levels  above  the 
sun's  surface  than  do  lines  less  intense  have  very 
important  consequences  in  the  problem  of  the  period  of  rota- 
tion of  the  sun.  The  determination  of  this  rotation  by  spec- 
troscopic  methods  is  one  of  the  very  greatest  perplexity 
and  still  awaits  an  adequate  solution.  The  information 
drawn  from  direct  observation  of  sun-spots  was  positive 
enough  as  far  as  it  went.  At  the  equator  on  the  sun,  spots 
take  about  twenty-five  days  to  make  a  complete  circum- 
ference, while  at  latitudes  north  and  south,  the  sun  rotates 
more  slowly ,  a  day  longer  being  required  for  a  spot  at  30° 
than  at  the  equator.  On  the  whole,  the  information  from 
spots  furnishes  contradictory  conclusions  since  the  indi- 
vidual spots  have  peculiar  motions  of  their  own  which  are 
not  representative  of  the  general  surface  of  the  sun.  Few 
spots  appear  at  distances  greater  than  40°  from  the  sun's 
equator,  and  manifestly  it  is  impossible  to  determine  the 
law  of  rotation  in  high  latitudes  where  observations  on  spots 
are  impossible. 

With  the  application  of  the  spectroscope  and  the  photo- 
graphic plate  to  the  problem,  it  was  confidently  expected  that 
all  difficulties  would  disappear,  since  the  measurements  were 
henceforth  to  be  made  on  sharp  and  well-defined  Fraunhofer 
lines.  All  that  seemed  necessary  for  a  complete  solution  of 
the  problem  was  to  utilize  a  spectroscope  of  sufficiently  high 
dispersion  so  that  the  shift  in  wave-length  due  to  rotation 
could  be  measured  with  an  adequate  degree  of  precision. 
The  great  advantage  of  the  spectroscopic  method  was  that 
the  observations  might  be  carried  out  at  any  time  without 
waiting  for  the  appearance  of  a  spot.  A  still  greater  ad- 

319 


320  ECLIPSES    OF    THE    SUN 

vantage,  however,  was  that  observations  were  not  confined 
to  the  zones  of  sun-spots,  but  could  be  pushed  even  to  the 
sun's  poles.  At  the  solar  equator,  the  eastern  limb  advances 
at  the  rate  of  2  .o  km  (1.2  miles )  per  second,  while  the  western 
limb  recedes  at  the  same  rate. 

From  observations  of  spots,  several  different  empirical 
formulas  have  been  devised  to  represent  the  rotation  of  the 
sun  at  different  latitudes.  Chief  among  them  may  be  men- 
tioned those  of  Carrington  and  Faye.  The  latter  has  usually 
been  regarded  as  the  most  satisfactory  formula,  and  to  it 
spectroscopic  observations  closely  conform.  Faye's  equation 
may  be  readily  adapted  for  spectroscopic  work  and  takes  the 
form  l 

v  +  Vi   =  (a  —  b  sin2/)  Cos  / 

where  v  is  the  velocity  in  the  line  of  sight  deduced  from  the 
actual  observations;  v\  is  a  correction  allowing  for  the  orbital 
revolution  of  the  earth  so  as  to  convert  synodic  periods  to 
sidereal ;  /  is  the  latitude  of  the  region  on  the  sun  under  in- 
vestigation; and  a  and  b  are  velocities  measured  in  kilo- 
meters (or  miles)  per  second.  It  has  been  assumed  that  a 
and  b  are  constants,  or,  in  other  words,  that  the  sun's  rota- 
tion is  not  varying.  If  b  is  equal  to  zero,  then  the  sun  would 
rotate  like  a  solid  sphere  with  equal  angular  rotations  in  all 
solar  latitudes. 

The  first  spectroscopic  observations  for  determining  solar 
rotation  are  more  than  forty  years  old,  and  were  made  by 
Duner  of  Upsala.  Two  lines  were  chosen  in  the  red  and 
their  wave-lengths  were  compared  with  terrestrial  lines  which 
consequently  show  no  rotational  shift.  The  measures  were 
carried  to  within  fifteen  degrees  of  the  poles  of  the  sun.  Ob- 
servations have  been  in  progress  continuously  for  more  than 
thirty  years.  The  values  for  the  first  decade  of  intensive 
work  have  furnished  J  the  basis  for  a  detailed  study  of  the 
problem. 

After  the  expenditure  of  so  much  time  and  energy  on 
this  research  it  must  be  confessed  that  the  results  attained 

1  Newall,  Monthly  Notices,  R.  A.  S.,  82,  JOT,  1921. 

2  St.  John,  Publ.  Astron.  Soc.  Pacific,  jo,  .$19,   1918. 


SUN-SPOT  GROUP  AND  PROMINENCE  PHOTOGRAPHED  AT  MT.  WILSON 
These  photographs  taken  in  the  red  light  of  hydrogen  show  exquisite  detail. 


5 


Q 

W 

I 


W 
U 


5 

O 


S! 

C4 

8 
I 

OH 
I 


HEIGHTS    AND    RELATED    PROBLEMS 


321 


are  a  grave  disappointment.  A  glance  at  the  velocities  at  the 
equator  given  in  the  table  will  reveal  values  ranging  in  size 
from  1.86  to  2 .08  km  per  sec.  The  measures  were  carried  out 
very  carefully,  using  every  precaution  to  free  them  from  er- 

LINKAR  VELOCITY  OF  SOLAR  ROTATION  AT  THE  EQUATOR 


Observe 

Location 

Velocity 

km  /sec 

No  of 
Lines 

Region 

Date 

Duner 

Upsala 

2   08 

2 

6301-6302 

1900  5 

Halm 

Edinburgh 

2    04 

2 

6301-6302 

1904 

Adams 

Mt.  Wilson 

2    06 

2O 

4196-4294 

1907 

Adams 

Mt.  Wilson 

2    05 

22 

4196-4291 

1908  5 

Storey  and  Wilson 

Edinburgh 

2    08 

10 

6280-6518 

1909 

Plaskctt,  J.  S. 

Ottawa 

2    01 

19 

5506-5688 

1911 

Plaskett,  J.  S. 

Ottawa 

2    O2 

T.S 

4196-4291 

191  1 

De  Lury 

Ottawa 

I    97 

19 

5506-5688 

1911 

Hubrecht 

Cambridge 

i    86 

40 

4299-4400 

igi  i 

Plaskett,  J   S 

Ottawa 

2    01 

27 

4250  and  5600 

1911-12-13 

Schlesinger 

Allegheny 

2    OO 

20 

4058-4276 

1912 

Even»hed  and  Kovds 

Kodaikanal 

I   05 

— 

3906  and  5624 

J9*3 

Plaskctt,  II.  II. 

Ottawa 

i    g8 

T  2 

5576-5628 

1913 

St.  John  and  Ware 

Alt   Wilson 

i   94 

35 

4123-4338 

1914 

Plaskett,  H   II. 

Ottawa 

1   95 

5 

5goo- 

1915 

St.  John  and  Ware 

Mt.  Wilson 

i   04 

26 

5018-5316 

1914-18 

St  John  and  Ware 

Mt.  Wilson 

i  95 

7 

6265-0337 

1916-17 

rors.  Each  of  the  results  in  the  above  table  is  the  mean  of 
a  very  great  number  of  observations  on  many  different  spec- 
trum lines  (in  one  case  numbering  forty),  and  it  would  seem 
as  a  consequence,  that  the  measures  should  have  a  high  de- 
gree of  precision  with  the  final  values  entirely  free  from  sys- 
tematic errors.  Taken  as  a  whole  the  measures  seem  to  prove 
conclusively  that  during  the  two  decades  of  observation  the 
sun's  speed  of  rotation  has  gradually  diminished,  showing  a 
total  change  of  five  per  cent.  Before  accepting  this  result  as 
the  truth,  we  must  not  forget  the  fact  that  the  wave-lengths 
of  lines  in  the  spectrum  have  not  the  constant  values  originally 
supposed  to  exist;  and  to  the  many  variations  already  known, 
eclipse  observations  add  another,  viz.,  that  the  stronger  spec- 
trum lines  occur  at  higher  levels  in  the  sun  and  must  therefore 
display  greater  velocities  of  rotation.  It  is  manifest  that 
if  the  observations  of  one  observer  are  to  be  compared  with 
those  of  another,  it  will  be  necessary  to  subject  the  lines  to 
a  number  of  refinements  as  follows:  ( i ) ,  Only  lines  with  well- 


322  ECLIPSES    OF    THE    SUN 

determined  wave-lengths  should  be  employed,  they  should 
not  exhibit  any  "  pole-effect,"  and  to  be  representative  lines 
they  should  not  be  "  enhanced;"  (2),  lines  of  like  Rowland 
intensities  only  should  be  used ;  ( 3 ) ,  lines  of  the  same  region 
of  the  spectrum  only  should  be  utilized  since  we  are  enabled 
to  see  into  the  sun  to  greater  depths  in  the  violet  than  in  the 
red,  this  being  due  to  the  scattering  of  light.  These  condi- 
tions will  limit  very  materially  the  choice  of  available  lines. 

Spectroscopic  observations  are  ordinarily  made  at  both 
ends  of  a  solar  diameter,  the  differential  measures  giving 
twice  the  value  of  the  displacement.  In  order  to  eliminate 
any  local  effect  at  the  limb  due  to  the  near  presence  of  spots, 
faculae  or  filaments,  it  would  be  well  to  compare  wave- 
lengths at  the  sun's  edge  with  those  at  the  center.  The  in- 
vestigations of  St.  John  1  show  that  the  wave-lengths  of  lines 
at  the  center  of  the  sun  are  "  constant  to  a  surprising  degree 
of  accuracy."  Newall  recommends  the  method  developed 
at  Cambridge,  of  making  the  measures  of  displacements  be- 
tween east  and  west  points  on  fixed  chords,  parallel  to  and 
equadistant  from  the  projected  axis  of  solar  rotation,  the 
east  and  west  points  being  chosen  in  the  same  heliographic 
latitude,  in  either  northern  or  southern  hemisphere.  This 
method  is  essentially  one  of  comparing  the  law  of  solar 
rotation  with  the  simple  law  of  uniform  rotation  of  a  solid 
body.  The  method  has  some  advantages  over  that  ordi- 
narily in  use,  and  some  disadvantages.  The  peculiarity  of 
this  method  is  that  the  spectra  are  taken  on  the  bright  parts 
of  the  sun's  surface  and  not  near  the  fainter  limb.  The 
work  of  De  Lury  has  shown  that  wave-lengths  determined 
at  the  limb  are  subject  to  slight  uncertainties  due  to  the 
superposition  of  the  sky  spectrum.  If  the  photographs  are 
taken  through  thin  haze,  the  wave-lengths  may  be  displaced 
by  amounts  depending  on  the  strength  of  the  solar  lines 
investigated. 

The  discussions  by  Newall 2  and  by  Halm  3  of  these  spec- 
troscopic  observations  have  led  to  interesting  conclusions. 
The  method  usually  followed  by  spectroscopists  is  to  secure 

1  Mt.  Wilson  Contributions,  No.  223,  1922.  3  Ibid.,  82.  479,  1922. 

-  Monthly  Notices,  R.  A,  S.,  82,  101,  1921. 


HEIGHTS    AND    RELATED    PROBLEMS          323 

observations  on  as  many  days,  at  as  many  different  solar 
latitudes  as  possible.  When  sufficient  material  is  obtained, 
the  velocities  in  the  line  of  sight  being  found,  they  are  then 
subjected  to  a  discussion  by  least  squares  in  order  to  deter- 
mine the  best  possible  values  of  the  constants  a  and  b  in 
Faye's  formula  above.  The  different  values  of  a,  the  velocity 
at  the  equator,  obtained  from  various  series  of  observations 
are  listed  in  the  table. 

It  might  be  said,  in  passing,  that  if  ever  in  the  history 
of  spectroscopic  work  positive  and  conclusive  results  were 
promised  by  any  piece  of  astronomical  research,  the  chances 
were  very  great  that  such  results  could  be  positively  assured 
beforehand  by  the  investigation  on  the  rotation  of  the  sun 
by  spectroscopic  methods. 

In  the  series  investigated  by  the  different  observatories, 
various  spectral  lines  and  regions  of  wave-lengths  were  em- 
ployed. In  spite  of  the  possible  sources  of  error  to  which  the 
measures  were  subject,  it  would  seem,  if  more  than  a  dozen 
or  fifteen  lines  were  measured  in  a  certain  series,  that  any 
local  peculiarities  of  a  single  line  should  be  certainly  aver- 
aged out.  From  the  nature  of  the  problem,  it  seemed  alto- 
gether probable  that  lines  of  approximately  the  same  aver- 
age Rowland  intensity  would  necessarily  be  chosen  by  the 
different  observers.  Hence  it  might  well  have  been  expected 
that  simultaneous  observations  of  the  sun  made  from  differ- 
ent observatories  would  thoroughly  agree  in  furnishing  the 
same  value  of  the  solar  rotation.  How  ideas  have  been 
changed  in  the  past  two  decades  regarding  the  constancy 
of  wave-lengths!  In  comparing  the  longer  series  of  ob- 
servations, we  have  the  results  tabulated  below,  where 
are  given  all  of  the  observations  secured  since  the  year 
1906  as  listed  by  Halm  in  Monthly  Notices.  In  addition 
to  the  individual  series  secured  at  Edinburgh,  Mt.  Wilson 
and  Ottawa,  there  is  given  the  mean  of  all  the  values  secured 
between  the  years  1901  and  1913,  and  also  the  results  for 
the  hydrogen  line,  Ha,  obtained  at  Mt.  Wilson.  The  veloci- 
ties are  given  in  kilometers  per  second  for  every  five  degrees 
of  heliographic  latitude. 

The  following  facts  should  be  noted  regarding  the  quanti- 


ECLIPSES    OF    THE    SUN 

ties  in  the  table:  ( i ),  the  values  derived  at  Edinburgh  agree 
perfectly  with  those  taken  almost  simultaneously  at  Mt. 
Wilson;  for  among  the  seventeen  separate  linear  velocities, 
only  two  differ  as  much  as  0.02  km  per  second,  showing  a  re- 
markable accord;  (2),  the  three  series  at  Ottawa  are  very 
consistent  throughout  all  latitudes;  (3),  the  three  Mt.  Wil- 
son series  found  in  the  third,  fourth  and  fifth  columns  agree 

LINEAR  VELOCITIES  OF  ROTATION  OF  THK  SUN 


Hclio- 

graphic 

IQ06   S 

iqo6  4 

1907  o 

1008  s 

IQTT  S 

iyi2  s 

101  *  S 

TQOI-I  \ 

1  008  S 

Lati- 

Ed 

Mt  VV 

Mt  W 

Mt   \V 

Ott 

Ott 

Ott 

Moan 

Ha 

tude 

0° 

2    03 

2   °5 

2    07 

2    00 

2    01 

2    00 

98 

2    04 

II 

5 

2    02 

2    03 

2    04 

2    04 

98 

98 

9<> 

2    02 

09 

10 

I    98 

T     QQ 

2    OO 

2    OO 

05 

05 

93 

99 

05 

*5 

1    Q2 

I    Q4 

I    Q4 

i   Q4 

89 

89 

88 

93 

OO 

20 

I     80 

I     87 

r   80 

j    80 

82 

83 

82 

80 

9? 

25 

I     78 

t     78 

i   77 

i    70 

74 

75 

74 

77 

83 

30 

t   09 

i   08 

i   00 

i   <>5 

04 

04 

02 

00 

73 

35 

i   53 

i   57 

*   54 

i   54 

53 

5-' 

50 

54 

i    Oi 

40 

i   44 

i   43 

T  41 

i   40 

40 

r    39 

37 

42 

1     ^0 

45 

I   30 

1     29 

'   27 

'    ~77 

20 

i    24 

•M 

i    28 

T   37 

50 

i    16 

i    is 

1    U 

I     I  2 

1  1 

I     1O 

09 

>4 

I     22 

5S 

r  02 

I    Ol 

0    C)Q 

o  97 

o  90 

o  05 

o  9S 

o  90 

1     08 

60 

o  88 

o  87 

o  85 

o  81 

o  8[ 

0    79 

o  80 

o  84 

o  9$ 

^5 

o  73 

o  73 

o  71 

o  07 

0    O/ 

O  0* 

o  65 

o  70 

o  78 

70 

o  60 

o  59 

O    0 

o  53 

c  52 

o  48 

o  50 

o  55 

o  03 

75 

o  45 

o  45 

o  43 

o  39 

o  >S 

°  35 

o  37 

o  41 

o  48 

80 

o  30 

o  30 

o   28 

O    20 

o  24 

O    22 

o  24 

o   27 

o  34 

perfectly  among  themselves  between  latitudes  o°  and  45°, 
but  show  a  progressive  diminution  of  velocities  at  all  latitudes 
poleward  of  45° ;  (4),  the  Mt.  Wilson  //a  values  are  greater 
in  amount  than  the  Mt.  Wilson  results  on  the  reversing  layer 
for  the  year  19064,  by  an  amount  which  is  approximately 
constant  and  equal  to  0.06  km  per  second ;  ( 5 ) ,  the  //a  values 
exceed  the  velocities  of  the  three  Ottawa  series  by  a  constant 
which  amounts  to  o.n  km  per  second;  (6),  the  time  of  ob- 
servations given  in  the  Edinburgh  and  in  the  first  Mt.  Wil- 
son column  (1906.4)  agrees  fairly  closely  with  sun-spot 
maximum,  while  the  mean  of  the  three  Ottawa  series  differs 
little  from  sun-spot  minimum. 

On  account  of  the, splendid  accord  between  the  observa- 


HEIGHTS    AND    RELATED    PROBLEMS          325 

tions,  we  seem  almost  forced  to  conclude  that  the  rotational 
velocity  of  the  sun  can  be  determined  spectroscopically  with 
great  exactitude.  The  results  tabulated  above  seem  to 
prove  most  emphatically  that  at  spot  maximum  the  revers- 
ing layer  revolves  with  a  speed  that  is  greater  than  the 
average,  while  at  minimum  of  spots  the  rotational  velocity 
is  less  than  the  average.  This  is  the  conclusion  independ- 
ently reached  by  Newall  and  by  Halm. 

A  decade  ago,  at  the  1922  meeting  of  the  International 
Astronomical  Union,  the  experts  were  forced  to  the  con- 
clusion that,  although  there  is  such  a  beautiful  accord  be- 
tween the  measures  listed  in  the  above  table,  it  may  not  be 
impossible  that  some  of  the  observations  may  have  been 
affected  by  errors,  either  accidental  or  systematic  in  nature, 
and  that  in  consequence,  the  difference  in  rotational  values 
at  maximum  and  minimum  of  spots  may  be  illusory  and  not 
real.  For  the  reasons  already  stated,  systematic  errors  due 
to  different  spectral  lines  investigated  may  readily  find 
place  in  any  series  of  measurements,  and  the  only  conclusion 
to  draw  from  the  large  differences  between  Mt.  Wilson  and 
Ottawa  is  that  one  or  both  of  these  series  of  measurements 
are  unquestionably  affected  by  systematic  errors.  In  fact, 
investigations  made  at  Mt.  Wilson,  in  which  simultaneous 
observations  are  made  upon  the  two  limbs  and  the  center 
of  the  sun,  indicate  that  temporary  and  local  conditions  fre- 
quently exist  in  the  reversing  layer  which  produce  differ- 
ences of  as  much  as  ten  per  cent  in  the  rotation  values 
obtained  by  comparing  east  and  west  limbs  directly.  Such 
results  make  it  obvious  that  misleading  values  may  be  actu- 
ally derived  from  a  short  series  of  observations;  in  fact  it 
may  be  well  to  ask,  along  with  the  Chairman  of  the  Com- 
mittee on  Solar  Rotation  of  the  International  Astronomical 
Union,  "  whether  the  solar  rotation  can  be  determined  as 
definitely  as  has  been  thought." 

One  obvious  method  of  testing  whether  systematic  errors 
affect  the  observational  values  and  whether  the  rotation  of 
the  sun  is  different  at  sun-spot  maximum  from  that  at  mini- 
mum is  for  one  observatory  with  fine  equipment  to  keep  at 
the  problem  for  a  long  stretch  of  years,  taking  great  care  to 


ECLIPSES    OF    THE    SUN 

keep  both  methods  and  observers  as  constant  as  possible. 
Hence  as  early  as  1914,  a  program  of  observations  was  inau- 
gurated at  Mt.  Wilson  Observatory  by  St.  John  and  Miss 
Ware.  Subsequent  to  the  values  given  in  the  table  above,  are 
the  following  linear  equatorial  velocities  '  at  the  sun's  equa- 
tor in  units  of  kms  per  second  : 


IQl6      igly      1918      1919      1920      1921      1922      192^      1()24      T92cS 

i   91          I   94          1.95          i   90         I   90          i   91          i   91          i   89          i   91          i   89 

More  recently,  St.  John2  obtained  values  in  1929  and  1930. 
That  in  the  latter  year,  amounting  to  1.95,  agreed  in  size  with 
those  of  1917  and  1918.  Still  more  recently,  Evershed*  ob- 
tained the  value  2.02  in  1931.  It  should  be  added,  however, 
that  Evershed  made  no  distinction  between  high-  and  low- 
level  lines  and  it  is  possible  there  may  be  a  systematic  error 
between  his  results  and  St.  John's. 

Following  the  method  of  Adams,  measures  of  rotation 
have  been  made  on  the  high-level  //a  of  the  chromosphere. 
At  Mt.  Wilson  in  1907,  the  value  was  2.11  km  per  sec.,  in 
1919  this  had  changed  to  2.00.  However,  in  1918,  Ever- 
shed  's  results  gave  2.07  km  per  sec.  while  at  Arcetri  in  1929 
the  value  2.07  was  also  obtained.  Again  we  are  faced  with 
the  conclusion  that  variations  in  instrumental  equipment 
might  readily  permit  different  levels  to  be  measured  at  two 
observatories  with  consequent  systematic  differences  in 
velocities.  The  simplest  method  of  explaining  the  large 
differences  in  the  Mt.  Wilson  values  is  to  assume  that  the 
measures  of  the  bright  line  of  Hex,  in  the  two  years  were  un- 
consciously referred  to  layers  at  different  heights. 

With  the  splendid  equipment  at  Kodaikanal,  measures 
have  been  carried  out  on  the  dark  Ha  line  of  the  Fraunhofer 
spectrum.  The  observations  in  the  years  1918-22  were  con- 
sistent and  numerous  and  a  value  for  the  linear  velocity  at 
the  equator  was  2.03  km  per  sec.  In  the  years  1923-29  the 
observations  were  not  so  numerous  and  they  yielded  a  slightly 
increased  value  of  2.05.  The  weighted  Kodaikanal  value  for 
the  whole  period  1918-29  is  2.03  km  per  sec.  During  the 

1  Mt.  Wilson  Obsy.,  Annual  Report  of  Director,  127,  1928. 

2  International  Research  Council,  Third  Report,  121, 

3  Monthly  Notices,  R.  A.  S.,  92,  105,  1931. 


HEIGHTS    AND    RELATED    PROBLEMS          327 

same  interval  at  Mt.  Wilson  the  value  from  different  lines 
was  quite  constant  at  1.90  km  per  sec.  On  account  of  the 
higher  level  at  which  the  Ha  line  has  its  origin,  the  results  of 
the  two  great  observatories  are  quite  consistent  in  showing 
during  a  fifteen-year  period,  1914-28,  a  solar  rotation  that 
appears  to  be  nearly  constant. 

In  the  latest  annual  report  (1931)  of  the  director  of  the 
Mt.  Wilson  Observatory,  the  intricate  problem  of  solar 
rotation  is  carefully  summed  up  in  the  following  words: 
"  It  has  become  increasingly  evident  that  the  rotation  of 
the  sun  as  determined  from  observations  of  the  reversing 
layer  is  not  constant.  Observations  by  Halm  at  Edinburgh 
and  Adams  at  Mt.  Wilson  gave  a  velocity  of  2.07  km  per 
sec.  and  a  minimum  period  of  24.75  days  for  1905.5,  while 
a  smaller  velocity  was  indicated  by  observations  at  Ottawa, 
Allegheny  and  Kodaikanal  for  the  epoch  of  1912,  For  the 
years  1919-29,  the  linear  velocity  averaged  1.90  km  per  sec., 
and  the  period  two  days  longer  than  in  1905.5.  Since  1928 
the  linear  velocity  has  slowly  increased,  while  the  period 
has  decreased  by  0.6  day.  On  the  other  hand,  the  velocity 
derived  from  long-lived  spots  has  remained  practically  con- 
stant at  2. 02  km  per  sec.  for  the  last  thirty  years.  The 
lower  reversing  layer  is  200  to  500  km  above  the  photosphere 
(level  of  spots).  In  1905.5  its  linear  velocity  was  0.06  km 
per  sec.  greater  than  that  of  the  photosphere,  and  in  1918, 
o.io  km  per  sec.  less,  thus  indicating  a  streaming  of  the 
lower  reversing  layer  relative  to  the  photosphere  from  east 
to  west  during  the  spot  cycle  1901-12,  and  from  west  to  east 
during  the  following  cycle.  If  connected  with  the  direction 
of  whirl  in  the  spot-forming  vortex,  this  systematic  differ- 
ence would  indicate  a  counter-clockwise  rotation  of  the  vor- 
tex in  the  northern  hemisphere  during  the  cycle  1901-13 
and  a  clockwise  rotation  during  the  following  cycle.  For 
the  cycle  beginning  in  1923,  the  linear  velocity  of  the  re- 
versing layer  might  have  been  expected  to  increase,  and 
apparently  it  has  increased ;  but  whether  it  will  continue  to 
do  so  remains  to  be  determined.  A  correlation  between 
linear  velocity  of  the  reversing  layer  and  direction  of 
whirl  in  the  spot-forming  vortex  would  be  of  great  inter- 


328  ECLIPSES    OF    THE    SUN 

est  in  its  bearing  on  the  sign  of  the  charge  in  the  spot 
vortex." 

A  summary  l  is  given  for  other  solar  phenomena  observed 
for  the  rotation  of  the  sun.  The  number  of  degrees  per  day 
at  or  near  the  sun's  equator  shown  from  observations  are: 
sun-spots,  14.39;  faculae,  14.49;  calcium  dark  filaments, 
14.45;  reversing  layer,  14.27;  Ha  (dark  marking)  14.40; 
Ha  (chromosphere,  Adams),  15.00;  K  (prominences)  17.1. 
To  find  how  the  rotation  periods  change  at  different  helio- 
graphic  latitudes,  one  should  also  consult  Abetti,  Handbuch 
der  Astrophysik,  4,  158,  1929. 

During  the  early  years  of  the  work  at  Mt.  Wilson,  the 
HOL  line  had  shown  such  different  rotational  values  from  the 
other  lines  that  it  seemed  advisable  to  extend  the  investi- 
gation to  other  solar  phenomena.  What  values  would  be 
given  by  flocculi?  To  photograph  in  the  red  required  special 
plates  sensitive  to  this  region.  When  the  flocculi  plates  were 
taken,  the  results  were  a  great  surprise  to  Hale  and  Adams, 
for  it  was  evident  at  a  glance  that  the  hydrogen  flocculi  dif- 
fered from  those  taken  by  means  of  the  H-  line  of  calcium 
in  several  remarkable  features."'  The  photographs  of  hydro- 
gen flocculi  already  secured  showed  that  they  did  not  share 
the  same  retardation  in  rotation  assumed  by  spots,  faculae 
and  calcium  flocculi.  But  the  striking  differences  between 
the  details  of  the  photographs  of  flocculi  in  the  HOL  light  of 
hydrogen  and  in  the  H^  line  of  calcium,  when  examined  side 
by  side,  showed  that  whereas  most  of  the  calcium  flocculi  are 
bright,  those  due  to  hydrogen  are  dark.  Further  differences 
were  exhibited  by  the  accentuated  definiteness  of  structure 
of  the  hydrogen  photographs  which  show  details  much 
smaller  in  size  and  with  greater  distinctness.  The  details 
brought  forth  by  the  Ha  line  were  more  marked  than  those 
obtained  by  the  other  hydrogen  lines  //(3,  //y  and  7/5,  the 
reason  for  the  difference  being  caused  by  the  greater  strength 
of  the  Ha  line  in  chromosphere  and  prominces  and  the 
greater  heights  attained.  A  most  unusual  series  of  photo- 
graphs was  secured  by  Hale  on  June  3,  1908,  showing  a  dark 

1  Kodaikanal  Bulletin,  No,  113,  1931. 

2  Mt.  Wihon  Contributions,  No.  26,  1908. 


HEIGHTS    AND    RELATED    PROBLEMS          329 

hydrogen  flocculus  which  was  actually  seen  drawn  into  a 
sun-spot  vortex,  the  remarkable  change  taking  place  within 
the  brief  space  of  time  of  ten  minutes.  The  day  previous, 
the  location  of  the  flocculus  was  evident  from  local  whirls. 
On  the  day  following  the  disappearance  of  the  mass  of  cool 
hydrogen  into  the  spot,  photographs  were  secured  showing 
eruptions  in  the  neighborhood  of  the  spot  due  to  bright  and 
hot  hydrogen  gas.  Apparently,  therefore,  a  sun-spot  is  a 
vortex  somewhat  resembling  a  terrestrial  cyclone.  Rela- 
tively cool  matter  in  the  gaseous  form,  floating  high  above 
the  solar  surface,  is  sucked  into  the  vortex  with  a  whirling 
motion.  After  sinking  into  the  interior  of  the  sun,  the  cool 
gas  is  heated,  and  later  in  the  heated  state  makes  its  re- 
appearance outside  of  the  limits  of  the  spot,  but  in  its  imme- 
diate neighborhood.  Meanwhile,  the  researches  of  J.  J. 
Thomson  and  others  had  shown  that  electrified  particles, 
both  positively  and  negatively  charged,  must  occur  in  vast 
numbers  in  a  hot  gaseous  body  like  the  sun.  If  there  is  a 
preponderance  of  positive  or  negative  charges,  their  rapid 
rotation  must  give  rise  to  a  magnetic  field.  In  other  words, 
a  sun-spot  by  its  vortex  motion  appears  to  create  its  own 
magnetic  field. 

The  most  promising  method  of  attack  appeared  to  be  the 
Zeeman  effect.  When  a  luminous  vapor  is  subjected  to 
a  magnetic  field  by  being  placed  between  the  poles  of  a 
powerful  magnet,  an  effect  is  produced  on  the  lines  of  the 
spectrum.  If  the  radiation  is  observed  along  the  lines  of 
force,  the  spectrum  lines  appear  in  most  cases  as  doublets, 
having  components  circularly  polarized  in  opposite  direc- 
tions. It  was  found  that  different  lines  of  the  same  element 
are  affected  to  a  different  degree,  and  that  the  distance  be- 
tween the  components  of  a  given  doublet  is  directly  propor- 
tional to  the  strength  of  the  field.  In  a  field  of  moderate 
strength  the  distance  between  the  two  components  of  a 
doublet  may  not  result  in  complete  separation,  the  lines  be- 
ing merely  widened,  while  in  other  lines,  which  are  excep- 
tionally sensitive,  the  separation  may  be  complete.  As  early 
as  1892,  Young  with  the  Princeton  refractor,  and  later 
W.  M.  Mitchell,  observed  lines  due  to  iron  in  the  sun  existing 


330  ECLIPSES    OF    THE    SUN 

as  single  lines  in  the  spectra  of  the  photosphere  but  double 
in  spot  spectra. 

If,  therefore,  the  observers  at  Mt.  Wilson  were  to  en- 
deavor to  find  a  magnetic  field  in  sun-spots,  the  line  of  at- 
tack was  clearly  outlined.  First  of  all,  there  was  necessary 
a  large  image  of  the  sun,  possible  only  by^the  use  of  a  tele- 
scope of  great  focal  length  so  that  the  surface  of  the  sun 
could  be  examined  in  regions  surrounding  the  spot.  The 
solar  light  must  then  be  examined  by  a  spectrograph  of  such 
great  power  that  the  separation  of  the  doublets  showing  the 
Zeeman  effect  might  be  as  great  as  possible.  To  interpret 
the  measures  of  the  solar  photographs,  investigations  in  the 
laboratory  unquestionably  would  be  necessary.  Fortunately 
for  the  development  of  astronomical  research,  the  resources 
of  the  Carnegie  Institution  were  ample  to  provide,  on  the  top 
of  Mt.  Wilson,  the  6o-foot  and  the  iso-foot  tower  telescopes 
with  powerful  spectrographs  attached,  and  to  equip  in  Pasa- 
dena a  physical  laboratory  with  the  necessary  forms  of  re- 
fined apparatus  and  Male's  spectrohelioscope.  As  the  result 
of  twenty-five  years  of  investigation,  the  scientific  informa- 
tion garnered  regarding  spots  and  the  general  magnetic  field 
of  the  sun  has  been  very  startling  and  most  important. 

Observations  are  secured  in  the  second  order  spectrum  of 
the  75-foot  spectograph  attached  to  the  iso-foot  tower  tele- 
scope. In  front  of  the  slit  of  the  spectograph1  is  placed  a 
Nicol  prism  in  combination  with  a  "  quarter-wave  plate  " 
built  up  of  mica  strips  2  mm  wide,  mounted  so  that  the  prin- 
cipal sections  of  successive  strips  make  an  angle  of  45°  with 
the  slit  and  90°  with  each  other.  If  the  long  axis  of  the 
Nicol  is  placed  parallel  to  the  slit,  the  mica  strips  will  alter- 
nately extinguish  the  red  and  the  violet  members  of  a  dou- 
blet, and  the  photograph  will  have  a  dentated  appearance 
shown  in  the  illustration  facing  page  332.  It  is  hardly  to  be 
expected,  however,  that  the  extinction  of  either  component 
will  be  complete.  A  partial  extinction  of  the  red  or  the  vio- 
let component  has  the  effect  of  shifting  the  maximum  of 
brightness  of  the  doublet  towards  the  red  or  the  violet  of  the 
average  position,  depending  on  whether  the  red  or  the  violet 

1  Hale,  Nature,  113,  105,  1924. 


HEIGHTS    AND    RELATED    PROBLEMS          331 

component  is  the  stronger.  When  the  two  components  are 
not  entirely  separated,  the  lack  of  uniformity  in  the  lines 
causes  a  displacement  which  depends  on  the  relative  strength 
of  the  two  components.  This  unsymmetrical  character  of 
the  lines  renders  the  accurate  measurement  of  the  displace- 
ments one  of  the  very  greatest  difficulty.  In  fact,  even  with 
the  great  scale  of  the  Mt.  Wilson  photographs,  reliable 
measures  are  possible  only  on  a  comparatively  small  num- 
ber of  lines. 

The  best  line  for  the  investigation  of  sun-spots  is  one  in 
the  red  at  6173.553  A.  This  line  is  split  into  three  compo- 
nents, the  two  outer  ones  being  elliptically  polarized  in  oppo- 
site directions,  the  inner  component  plane  polarized.  If  the 
red  component  of  the  line  is  transmitted  by  the  compound 
quarter-wave  plate  and  the  violet  component  extinguished, 
then  the  polarity  of  the  spot  is  N,  north-seeking  or  positive; 
while  if  the  violet  component  is  seen  the  polarity  is  S  or 
negative.  Since  the  distance  between  the  components  is 
proportional  to  the  strength  of  the  magnetic  field,  the  direct 
measurement  of  a  photograph  by  means  of  a  micrometer 
readily  furnishes  the  field-strength.  In  addition,  the  angle 
between  the  magnetic  lines  of  force  and  the  solar  surface 
can  be  determined  in  all  parts  of  a  spot. 

Ever  since  the  time  of  Galileo  it  has  been  noticed  that 
spots  have  a  tendency  to  appear  in  pairs.  In  fact,  the  ob- 
servations at  Mt.  Wilson  show  only  that  in  ten  per  cent  of  all 
cases  are  the  spots  single  or  unipolar  in  appearance.  Sixty 
per  cent  of  spot  groups  are  distinctly  bipolar,  while  the  bal- 
ance of  thirty  per  cent  show  a  tendency  towards  the  bi- 
polar type,  exhibiting  calcium  or  hydrogen  flocculi  following 
or  preceding  a  single  spot.  In  many  bipolar  groups,  one  of 
the  members,  usually  the  preceding  or  western  spot,  is  much 
larger  than  the  other.  In  nearly  all  cases  it  has  been  found 
that  the  eastern  member  of  a  spot  pair  differs  in  polarity 
from  the  western  member.  When  the  group  is  formed  of  a 
number  of  spots  small  in  size  but  without  any  dominant 
members,  then  the  groups  of  spots  at  opposite  ends  of  the 
stream  are  usually  opposite  in  polarity. 

According  to  the  opinion  of  Hale,  a  spot  is  a  vortex  in 


332  ECLIPSES    OF    THE    SUN 

which  the  solar  gases  are  expanded ,  and  consequently  cooled, 
by  centrifugal  action.  If  this  cooling  is  sufficient,  the  gases 
appear  darker  over  the  vortex  because  of  their  decreased 
radiation  and  increased  absorption  at  reduced  temperatures. 
In  the  cooler  regions,  chemical  compounds  are  formed,  such 
as  titanium  oxide  and  certain  hydrides,  and  their  pr^ence 
is  recognized  by  the  existence  of  characteristic  bands  in  sun- 
spot  spectra.  It  is  quite  possible,  however,  that  the  cooling 
may  not  have  advanced  sufficiently  far  for  the  spot  to  become 
visible  as  a  darkened  area  on  the  surface  of  the  sun,  but 
nonetheless  it  has  been  found  that  the  magnetic  effects  per- 
sist even  after  the  spot  ceases  to  be  visible  and  also  appear 
before  the  darkening  of  the  spot  becomes  sufficiently  great 
to  bring  it  into  view.  Thus  have  been  discovered  "  invisible 
spots/'  with  the  result  that  the  life  history  of  a  spot  is  in- 
creased by  lengthening  out  the  times  of  observation  both 
before  and  after  the  spot  becomes  visible  to  the  eye. 

Ever  since  the  beginning  of  the  work  at  Mt.  Wilson  in  the 
year  1908,  many  thousands  of  spots  have  been  observed  with 
great  care.  It  has  been  found  that  all  spots,  independent  of 
size,  contain  magnetic  fields,  but  that  the  field-strengths  up 
to  a  certain  maximum  increase  with  the  diameter  of  the  spot. 
Since  the  spots  show  themselves  to  be  gigantic  vortices,  a 
natural  question  to  ask  is  whether  the  directions  of  the 
whirls  follow  a  law  similar  to  cyclones  and  tornadoes  on  our 
earth  which  move  in  directions  contra-clockwise  in  the  north- 
ern hemisphere  but  right-handed  in  the  southern  hemisphere. 
Little  is  known  regarding  the  sign  of  the  charge  dominant  in 
sun-spots,  but  if  it  is  assumed  that  these  charges  are  the 
same  in  all  solar  vortices  then  the  observed  polarities  will 
give  the  directions  of  the  whirls.  It  has  been  found  that 
preceding  spots  in  the  northern  and  southern  hemispheres  of 
the  sun  exhibit  opposite  polarities  and  consequently  opposite 
directions  of  whirl. 

What  is  the  law  underlying  the  magnetic  changes  in  sun- 
spots?  From  the  time  of  the  inception  of  the  work  at  Mt. 
Wilson  in  June,  1908,  to  the  minimum  of  spots  in  December, 
1912,  there  were  a  total  of  twenty-six  groups  observed.  In 
seven  of  these  groups  that  appeared  in  the  northern  hemi- 


?lf;^^^^fepll^ 

'i  '"   --'''•"  v.'n'  ' ., ; j-    ny-to'*?1**  nH  -V  Vw=  ^i  i;  '  -\J'""' "'  '\t~'  ~  "-'  ''••>'{--  '•   •-   ,  "f  fl" 

rlr --^^^V'*^:? '^ -;  $& »#':_&,^ ^:;;>j?'_    ;J  ' 


^-       ' 


ZEEMAN  EFFECT  IN  SUN-SPOTS 

a,  6,  ^  Photographic  observations  of  a  multipolar  sun-spot  group  on  August  8, 
1917  at  Mt.  Wilson  Observatory;  b  and  c  photographed  with  Nicol  and  quarter- 
wave  plate  show  the  Zeeman  effect  with  the  iron  triplet  6173  A.  d  An  en- 
larged direct  photograph  of  the  same  spot  group  on  August  8,  1917  with  the 
6o-foot  tower  telescope.  (The  black  disk  in  corner  shows  the  size  of  the  Earth.) 


s 

2 


i 

o 

£ 


a  * 

£  a 

si 

^  31 
- 


IS 


in   +j 


2 


5 

£ 


HEIGHTS    AND    RELATED    PROBLEMS         333 

sphere,  the  polarity  of  the  preceding  member  of  the  pair, 
characterized  by  the  violet  member  of  the  6 1 73-line,  was 
south-seeking  or  negative,  and  that  of  the  following  spot 
was  north-seeking  or  positive.  In  the  southern  hemisphere, 
for  seventeen  groups  the  polarities  were  the  reverse  of  those 
in  northern  latitudes,  N  or  positive  for  the  preceding  mem- 
ber, and  S  or  negative  for  the  following  member  of  the  pair. 
Two  southern  spots  out  of  the  total  of  twenty-six  observed 
showed  opposite  polarities  to  those  expected,  thereby  fur- 
nishing exceptions  to  the  rule. 

The  spots  which  were  observed  were  the  dying  members 
of  the  spot  cycle  and  had  an  average  latitude  of  9°.    Would 

N40i 


30 
10 


u, 
o 

D 
>-      0 

3,0 


zo 


$40 


FIG.  7    Sun-Spot  Polarities  observed  at  Mt.  Wilson  between  1908  and  1925. 

The  curves  represent  the  change  in  latitudes  of  the  spots;  the  preceding 
spot  is  shown  on  the  right. 

the  spots  appearing  in  the  new  cycle,  in  high  northern  and 
southern  latitudes  continue  the  same  persistence  of  polarities 
that  the  spots  had  already  exhibited?  There  appeared  no 
reason  for  a  change,  but  when  the  observations  were  obtained 
they  furnished  a  great  surprise  in  showing  a  complete  re- 
versal of  polarities  from  those  observed  in  the  previous 
cycle. 

During  the  eleven-year  period  which  closed  in  1923  more 
than  two  thousand  spot  groups  were  observed  at  Mt.  Wilson. 
Unipolar  and  bipolar  groups,  with  only  four  per  cent  of  ex- 
ceptions, continued  to  have  the  polarities  reversed  from  that 
of  the  preceding  cycle  and  showed  south-seeking  or  negative 
polarities  in  the  northern  hemisphere,  and  north-seeking  or 


334 


ECLIPSES    OF    THE    SUN 


positive  polarities  for  the  preceding  spots  of  the  southern 
hemisphere. 

Observations  after  the  spot  minimum  in  1923  were  anx- 
iously awaited  to  know  whether  a  reversal  would  again  take 
place  with  a  resumption  of  polarities  that  had  been  the  rule 
in  the  second  preceding  cycle.  The  answer  was  a  very  con- 
clusive one,  the  reversal  which  was  now  expected  had  actu- 
ally taken  place. 

The  law  of  sun-spot  polarities  is  expressed  in  the  two 
appended  figures.  At  sun-spot  minimum  the  new  cycle  origi- 
nates in  high  northern  and  southern  heliographic  latitudes. 


FIG.  8     Sun-Spot   Polarities  at    Minimum   of   Solar   Activity. 

Two  zones  in  each  hemisphere,  in  which  the  ^pots 

arc  of  opposite  magnetic  polarity,  exist  for  about  two 

years  near  spot  minimum. 

As  the  cycle  progresses  towards  maximum  of  spots  they  draw 
closer  to  the  sun's  equator  but  the  polarity  remains  always 
the  same.  The  high-latitude  spots  of  each  cycle  have  op- 
posite polarities  from  those  of  the  preceding  cycle.  A  strange 
condition  appears  near  minimum  of  spots.  In  the  northern 
(or  southern)  hemisphere  two  groups  of  spots  differing  a  few 
degrees  in  latitude  may  exhibit  opposite  polarities,  one  po- 
larity due  to  a  spot  in  the  waning  cycle,  the  other  taking  its 
sign  from  the  cycle  just  beginning.  Since  the  directions  of  the 
whirls  of  the  hydrogen  flocculi  are  not  reversed,  but,  on  the 
contrary,  there  is  a  change  in  the  sign  of  polarity  at  each  spot 


HEIGHTS    AND    RELATED    PROBLEMS  335 

minimum,  it  is  evident  that  the  magnetic  period  of  spots  is 
22  years,  or  twice  the  n-year  period  on  which  the  numbers 
of  spots  depend. 

It  is  evident  that  the  general  magnetic  field  of  the  sun 
must  be  much  weaker  than  that  exhibited  in  sun-spots  on 
account  of  the  fact  that  the  sun  as  a  whole  rotates  much 
more  slowly  than  the  vortex  whirlpools  of  the  spots.  More- 
over, in  attempting  to  secure  the  magnetic  field  of  the  sun  as 
free  as  possible  from  any  local  effect  of  sun-spots,  it  is  obvi- 
ously necessary  to  obtain  photographs  when  the  sun  is  en- 
tirely free  from  active  spots.  Several  series  of  photographs 
taken  under  satisfactory  conditions  have  been  secured.  The 
difficulty  of  measuring  these  photographs,  and  of  securing 
these  measures  free  from  personal  and  systematic  errors,  is 
patent  to  anyone  who  has  ever  engaged  in  any  astronomical 
measurements  requiring  the  setting  on  two  close  components 
of  a  double.  Moreover,  the  size  of  the  quantity  to  be  meas- 
ured is  only  o.ooi  A.  As  a  matter  of  fact,  van  Maanen  was 
the  only  one  of  the  five  Mt.  Wilson  measurers  that  engaged 
in  the  work  whose  results  had  a  satisfactory  degree  of  con- 
sistency. On  account  of  the  difficulties  in  measuring,  lines 
having  an  intensity  greater  than  5  in  the  solar  spectrum  and 
weaker  than  o  had  to  be  excluded. 

The  general  summary  of  the  results  seems  to  prove  con- 
clusively that  a  general  magnetic  field  exists  in  the  sun,  and 
that  in  consequence  the  sun  behaves  approximately  as  a 
uniformly  magnetized  sphere,  with  the  magnetic  axis  only 
slightly  inclined  to  the  solar  axis  of  rotation,  and  with  a 
polarity  corresponding  to  that  of  the  earth.  Forty-six  spec- 
tral lines  were  investigated,  and  of  these,  30  lines  due  to  ¥e, 
Cr,  Ni,  V  and  Ti  show  displacements.  The  strength  of  the 
magnetic  field  determined  for  each  line  showed  a  correlation 
between  field-strength  and  line  intensity,  the  stronger  fields 
being  connected  with  lines  of  smallest  intensity.  Eclipse  spec- 
tra reveal  the  information  that  the  weakest  solar  lines  origi- 
nate at  the  lowest  depths.  Since  it  was  possible  to  measure, 
for  the  determination  of  the  strength  of  field,  lines  of  a  solar 
intensity  5  or  less,  it  is  manifest  that  lines  only  in  a  very  shal- 
low layer,  less  than  450  km  in  depth,  can  show  the  influence  of 


336  ECLIPSES    OF    THE    SUN 

the  sun's  magnetic  field  large  enough  to  be  detected  by  the 
present  method  of  attack.  The  period  of  rotation  of  the 
sun's  magnetic  axis  was  found  to  be  31.52  days.  No  explana- 
tion can  be  given  for  this  peculiar  value  of  rotation  which 
differs  in  such  marked  degree  from  the  equatorial  value  de- 
rived from  sun-spots.  In  fact,  no  adequate  explanation  is 
yet  forthcoming  to  explain  the  cause  of  the  sun's  magnetism. 

It  is  now  generally  conceded  by  all  scientists  that  the  sun 
is  continually  sending  off  a  vast  stream  of  negatively  charged 
particles,  or  electrons.  It  is  also  generally  conceded  that 
the  presence  of  spots  on  the  sun  is  evidence  of  great  solar 
activity.  If  the  spot  is  large,  and  the  activity  consequently 
great,  the  streams  of  electrons  directed  toward  the  earth 
may  reach  our  upper  atmosphere  in  vast  numbers.  In  the 
rarefied  conditions  that  must  exist  in  the  upper  atmosphere, 
the  air  becomes  ionized.  This  ionization  causes  the  electro- 
magnetic display  known  as  northern  lights,  or  aurora  bore- 
alis.  As  a  result  of  the  electrification  of  the  atmosphere, 
currents  are  induced  in  the  earth,  which  may  at  times  be  so 
strong  as  to  seriously  interfere  with  the  sending  of  messages 
over  the  telegraph  lines  or  submarine  cables.  As  a  further 
manifestation  of  the  currents  flowing  through  the  earth,  the 
navigator's  compass  may  be  affected  with  the  result  that  the 
north  end  of  the  needle  may  point  several  degrees  from 
the  magnetic  north.  This  leads  to  a  so-called  "  magnetic 
storm."  The  display  of  a  brilliant  aurora  is  very  frequently 
accompanied  (and  caused)  by  a  spot  central  on  the  face  of  the 
sun.  On  May  13,  1921,  the  spots  near  the  sun's  center 
caused  an  unusually  gorgeous  display  of  aurora  visible  prac- 
tically all  round  the  world.  The  telegraph  lines  were  seri- 
ously hampered  in  the  sending  of  messages.  While  the 
aurora  was  at  its  height,  one  of  the  Atlantic  cables  connect- 
ing Europe  and  America  was  burnt  out.  Whether  this  was 
caused  by  the  earth's  currents,  or  was  but  a  strange  coinci- 
dence, has  never  been  fully  determined. 

Theoretically,  sun-spots  should  possess  an  electric  as  well 
as  a  magnetic  field.  Although  observations  have  been  made 
at  Mt.  Wilson  and  elsewhere,  no  conclusive  evidences  of  a 
Stark  effect  have  been  recorded. 


HEIGHTS    AND    RELATED    PROBLEMS  337 

In  an  important  paper  presented *  at  the  National  Acad- 
emy of  Sciences,  Adams  and  Nicholson  discuss  The  Nature 
of  the  Solar  Cycle  in  the  following  manner.  "  The  eleven- 
year  period  may  vary  between  nine  and  fourteen  years  and 
the  amplitude  of  the  cycle  by  about  50  per  cent  of  its  average 
value.  Period  and  amplitude  are  apparently  unrelated.  It 
seems  probable  that  both  the  quantity  and  quality  of  solar 
radiation  vary  during  this  cycle  and  many  attempts  have 
been  made  to  correlate  terrestrial  phenomena  with  sun-spots. 
Definite  correlations  have  been  found  with  the  variations  in 
terrestrial  magnetism  and  its  related  phenomena.  There  is 
evidence  of  a  slight  correlation  between  sun-spots  and  atmos- 
pheric temperature  in  certain  regions  on  the  earth  and  with 
other  factors  of  weather  and  climate  for  limited  regions  and 
for  limited  time  intervals.  These  correlations  are  so  uncer- 
tain that,  in  the  majority  of  cases  at  least,  predictions  on 
them  have  very  little  weight." 

1  Science,  75,  594,  1932. 


CHAPTER  XIX 

HEIGHTS  AND  DISTRIBUTIONS  OF  VAPORS 

NEW  information  gleaned  in  the  past  decade  about  the 
structure  of  the  atom  has  revolutionized  the  method 
of  attacking  solar  problems.  Practically  all  of  the 
prominent  lines  in  the  spectrum  of  the  sun  have  been  assigned 
to  multiplets  with  known  excitation  potentials,  the  arbitrary 
intensity  scale  of  Rowland  has  been  submitted  to  calibration 
tests  which  have  revealed  that  the  intensities  depend  on  the 
number  of  atoms  engaged  in  the  formation  of  the  spectral 
lines.  From  the  weakest  lines  perceptible  in  the  solar  spec- 
trum of  intensity  -  3  ( or  oooo )  to  the  strongest  Fc  lines  at 
wave-length  3720  and  3735  of  intensity  40,  the  number  of 
atoms  involved  increases  about  one  million  times. 

A  person  having  no  knowledge  of  the  theory  underlying 
multiplet  groups  would  not  advance  very  far  in  the  practical 
operation  of  correlating  heights  in  the  chromosphere  with  in- 
tensities either  in  sun  or  chromosphere  before  the  fact  would 
be  forced  upon  his  attention  that  generally  the  lines  of  great- 
est intensity  reach  the  greatest  heights,  and  moreover  the  in- 
tensities and  heights  for  any  element  are  greatest  for  the 
multiplets  of  lowest  excitation  potential.  The  best  element 
for  a  study  of  the  explanation  of  these  correlations  is  unques- 
tionably neutral  Fe.  This  element  has  been  assiduously  ob- 
served in  the  laboratory  and  very  exact  wave-lengths  are 
known.  Fe  is  very  rich  in  lines  which  have  been  grouped  into 
multiplets  with  a  wide  range  of  excitation  potentials.  Some 
of  the  more  important  results  from  the  detailed  study x  of  this 
element  are  given  herewith.  The  lowest  atomic  level  of  neu- 
tral Fe  is  the  aGD  level  with  average  excitation  potential  of 
0.07  volts,  the  next  lowest  level  is  a~'F,  with  excitation  poten- 
tial of  0.92  volts.  The  table  gives  the  average  heights  in  the 

1  Publ.  Leander  McCormick  Ob\y.f  5,  114;  Astroph.  Jour.,  72,  146,  1930. 

338 


HEIGHTS  AND  DISTRIBUTIONS  OF  VAPORS     339 

chromosphere  attained  by  lines  of  different  Rowland  inten- 
sities, with  the  whole  material  arranged  according  to  in- 
creasing excitation  potentials.  A  total  of  789  lines  is 
involved. 

EXCITATION  POTENTIALS,  ROWLAND  INTENSITIES  AND  HEIGHTS* 


Excitation  Potent  a 

20   jo 

12-15 

TO 

7-Q 

6 

5 

4 

, 

2 

i 

o 

0.07         .     . 

J2000 

I       4 

1625 

900 

4 

1  100 
II 

1075 
4 

6OO 

5 

517 

3 

428 

7 

444 
8 

350 
3 

0.92       . 

[1300 

I       7 

1225 

4 

917 
3 

15 

638 

8 

ft28 

ii 

504 

12 

442 
13 

400 
i 

333 
3 

/i  650 

I           2 

4 

1075 
4 

875 
ft 

ftoo 

2 

500 
3 

435 
13 

4T7 
9 

383 
3 

2  18-249    .    . 

/   775 

\       4 

(>38 
10 

24 

524 
17 

467 
17 

419 

27 

403 
16 

8 

2  50-2  99 

/   500 
1        i 

i  5 

537 

493 

479 

38 

433 

412 

24 

329 

12 

300 

2 

300-3.49 

/  542 
I       0 

450 
9 

447 

18 

440 

21 

425 
26 

3<H 

21 

325 

6 

300 

I 

3.50-3.99    

i   "I 

400 
4 

421 

7 

373 

387 
15 

375 
H 

350 
5 

3OO 
2 

4  00-4  49     ... 

/     4°° 

1              i 

400 
2 

4^5 

10 

390 
10 

372 

339 
32 

321 
17 

250 

7 

450-499  

/    4°O 

I       4 

383 
ft 

400 
ft 

394 
9 

333 
3 

300 

2 

*  The  upper  quantity  give*  the  height  in  kilometers,  the  lower  gives  the  number  of  spectral  linea  involved 

The  following  facts  should  be  noted:  (i)  The  strongest 
lines  of  neutral  Fe  in  the  sun  belong  to  multiplets  of  lowest 
excitation  potential  0.07  volts.  (2 )  With  increase  of  excita- 
tion potential,  the  maximum  intensity  of  the  lines  in  the  mul- 
tiplets steadily  decreases.  ( 3  )  For  multiplets  of  any  given 
excitation  potential,  there  is  a  close  correlation  between  in- 
tensities and  heights.  (4)  For  any  given  Rowland  intensity, 
such  as  6,  the  heights  diminish  (vertically  in  the  column)  as 
the  excitation  potentials  are  increased.  To  know  the  height 
corresponding  to  a  line  of  intensity  6,  the  value  of  the  excita- 
tion potential  is  necessary. 

If  the  material  from  the  flash  spectrum  on  neutral  Fe  is 
divided  at  wave-length  4900  A  into  two  groups,  we  have  the 


340 


ECLIPSES  OF  THE  SUN 


information  in  the  following  table.  The  first  two  horizontal 
lines  of  the  table  give  the  average  heights  and  average  ex- 
citation potential  of  the  lines  to  the  violet  of  4900  A  ar- 
ranged according  to  their  Rowland  intensities.  The  next 
two  lines  give  similar  data  from  the  lines  to  the  red  of 
4900  A;  and  next  comes  the  average  for  all  lines  of  all  wave- 
lengths. The  two  last  lines  in  the  table  give  the  radial  mo- 
tion in  the  penumbra  of  sun-spots  called  the  Evershed  effect, 
measured  by  St.  John  at  Mt.  Wilson,  the  first  line  giving  the 
value  in  angstroms  and  the  second  line  in  kilometers  per 
second. 

ROWLAND  INTKNSITFKS,  HKIGHTS,  AND  EXCITATION  POTENTIALS 


0 

I 

2 

3 

4 

5 

0 

7 

8 

9 

/    3>I 

372 

WO 

^02 

409 

SOI 

570 

Oo  >> 

720 

(»8i 

9 

i  2   20 

2  95 

I  oi 

2   70 

2.02 

2  54 

2   29 

2   2J 

i  82 

r  90 

I 

1     287 

ioS 

iio 

i02 

40S 

40  s 

150 

5'4 

S^S 

Ooo 

ft 

I  3  bo 

3  44 

304 

3-13 

i  U 

3  10 

2  57 

2  OS 

I  82 

2    }2 

2 

I    290 

Hg, 

30  1 

122 

150 

uo 

SS2 

570 

08  1 

917 

H 

\  3  H 

3-23 

*  *5 

2  90 

290 

2  77 

2  39 

2    ^0 

i  82 

198 

r 

030 

028 

.025 

025 

.021 

.019 

oiO 

013 

OIO 

.005 

( 

\20 

1.7 

i  5 

I  4 

I   2 

i.i 

I  0 

08 

oO 

03 

o 

971 

T  SO 


OO2 
O  1 


IS  18 


.001 

O  1 


20-40 


75 


OOO 

o  o 


Early  in  the  year  1909,  Evershed  announced  a  remarkable 
discovery  of  far-reaching  importance  in  his  observations  of 
the  displacement  of  Fraunhof er  lines  in  the  penumbra  of  sun- 
spots.  With  the  slit  of  his  spectograph  placed  across  the 
spot,  he  found  that  the  wave-lengths  of  lines  in  the  penumbra 
of  the  spots  were  different  from  the  values  at  the  center  of  the 
sun.  The  displacements,  which  affected  practically  all  of  the 
lines  of  the  reversing  layer,  were  not  constant  but  differed  in 
amount  depending  on  the  intensity  of  the  lines  investigated. 
The  shift  was  greater  for  the  weaker  lines  of  the  spectrum 
than  it  was  for  the  stronger  lines.  Evershed  advanced  the 
hypothesis  that  the  observed  displacements  are  the  result  of 
the  Doppler  effect,  and  that  in  consequence,  the  gases  of  the 
reversing  layer  are  in  radial  motion  tangential  to  the  solar 
surface.1 

1  Kodiakanal  Observatory  Bulletin,  No,  XV:  Kodiakanal  Observatory  Memoirs. 
r,  Pt.  i. 


w 


5 


0 


. 


Q 

a 


HEIGHTS  AND  DISTKIHTTIONS  OF  VAPORS     341 

Following  the  announcement  of  this  important  discovery, 
St.  John  began  an  extended  series  of  investigations  into  the 
subject,  the  results  of  the  observations  being  published  in  the 
Contributions  from  the  Mount  Wilson  Observatory,  Nos.  69, 
74, 88,  348  and  390.  The  observations  were  carried  out  with 
the  6o-foot  tower  telescope,  with  the  image  of  the  sun  170 
mm  in  diameter,  the  penumbra  of  the  spots  investigated  aver- 
aging 3.0  mm  in  diameter.  The  plates  in  the  violet  and  green 
were  taken  in  the  third  order  spectrum,  and  those  in  the  yel- 
low and  red  in  the  second  order.  For  the  two  cases,  the  dis- 
persion was  i  mm  =  0.56  A  and  i  mm  =  0.86  A,  respectively. 
Measures  were  carried  out  on  506  lines,  some  of  the  lines  be- 
ing measured  on  no  less  than  thirty  plates. 

The  Mt.  Wilson  measurements,  so  carefully  made  by  Miss 
Ware,  abundantly  verified  Evershed's  conclusions  that  the 
displacements  are  caused  by  movements  of  the  solar  vapors 
tangential  to  the  solar  surface  and  radial  to  the  axis  of  the 
spot.  These  motions  are  none  other  than  the  actual  flow  of 
the  material  of  the  reversing  layer  out  of  the  spots  and  of 
the  matter  forming  the  chromosphere  into  the  spot  vortex. 

The  layers  closest  to  the  sun's  surface  have  a  motion  of 
translation  out  of  the  spot  at  the  rate  of  two  kilometers  per 
second,  and  this  motion  becomes  less  and  less  at  greater  and 
greater  elevations  until,  at  a  height  of  about  two  thousand 
kilometers,  the  motion  outward  of  gases  from  the  sun-spot 
ceases.  What  happens  to  the  vapors  above  this  level?  The 
information  furnished  by  the  investigations  of  St.  John  is  very 
definite  and  apparently  admits  of  no  contradiction.  From 
measurements  on  several  elements,  it  was  found  that  above 
this  level  of  inversion,  the  gases  of  the  chromosphere  take  a 
motion  carrying  them  in  to  the  spot.  As  greater  and  greater 
elevations  are  reached  these  movements  increase  in  amount. 
At  the  maximum  heights  reached  by  lines  of  the  chromo- 
sphere, which  are  attained  only  by  the  H  and  K  lines  of  the 
element  Ca,  there  is  a  movement  of  the  calcium  vapor  into 
the  spot  at  a  speed  of  3.8  km  per  second  corresponding  to  a 
displacement  measured  by  St.  John  of  -0.063  angstroms. 

In  the  following  table  there  is  collected  the  available  in- 


342 


ECLIPSES  OF  THE  SUN 


formation  from  the  measures  of  heights  derived  from  the 
lines  in  the  flash  spectrum  and  of  displacements  due  to  the 
Evershed  effect  from  506  lines  in  the  penumbra  of  sun-spots. 
The  +  symbol  affixed  to  an  element,  as  Ca  +,  signifies  that 
the  line  in  the  spectrum  is  enhanced  and  that  it  takes  its  origin 
from  the  ionized  atom.  The  conclusions  of  St.  John  (loc. 
cit.}  are,  "  In  the  observation  of  these  velocities  we  have  a 
methol  of  sounding  the  solar  atmosphere  and  of  allocating 
the  relative  levels  of  the  lines." 

MOTIONS  OF  SOLAR  GASKS  AT  VARIOUS  LKVELS 


Currents  measured  in  Kms/sec 

Heights  in  Kms 
from  Flash  Spectra 

Element 

Spots 

Disk 

14,000 

Ca+  (II  and  K) 

3  8  inwards 

o  5  downwards 

I2,OOO 

Ha 

30 

8,500 

77/3,  77*,  77f 

8,OOO 

777,  776 

i   i 

7,500 

He 

7,000 

M* 

o  7        ' 

6,000 

77+,Sr+,kVr+ 

o  5 

5,000 

Ca  (4^27) 

O    2            * 

2,000 

Al  (15-20) 

O    O 

i  ,500 

Fe  (15-40) 

o  i  outwards 

o  3  downwards 

1,000 

Fe  do) 

0    2 

600 

Fe  (8),  TV,  Sc,  V 

o  6 

O    0 

400 

Fe  (4),  0,  Sr,  V 

I     2 

350 

Fe  (2),  A7,  Co,  Mn 

i   5 

300 

Fe  (i) 

i   7 

o  3  upwards 

275 

Fe  (oo) 

2    O 

It  is  evident,  from  a  glance  at  the  above  table,  that  the 
enhanced  lines  not  only  extend  to  greater  elevations  than 
do  the  unenhanced  lines  of  the  same  element,  but  that  the 
Evershed  effect  for  them  is  more  pronounced  as  well. 

Eclipse  spectra,  taken  in  connection  with  the  Evershed 
displacements,  can  shed  some  light  on  the  problem  of  sun- 
spots,  not  only  regarding  the  time-honored  question  of  the 
heights  above  the  photosphere  at  which  the  spots  themselves 
originate  but  also  regarding  the  magnificent  work  of  Hale  on 
spot  vortices.  After  long  years  of  uncertainty  regarding  the 
effective  temperature  of  spots,  whether  they  are  hotter  or 
cooler  than  the  photosphere,  we  have  finally  come  to  the  con- 


HEIGHTS  AND  DISTRIBUTIONS  OF  VAPORS     :j*;j 

elusion  that  the  spots  are  relatively  cooler.  This  fact  first 
became  known  through  the  discovery  by  Fowler  of  bands  in 
the  red  part  of  the  spectrum  due  to  titanium  oxide.  Bands 
and  flutings  of  hydride  compounds  have  likewise  been  found 
in  parts  of  the  spectrum  other  than  the  red.  There  seems 
only  one  explanation  possible,  which  is  that  the  compounds 
can  exist  only  at  the  cooler  temperature  of  the  spots,  and  that 
at  the  higher  temperatures  of  the  photosphere  the  com- 
pounds are  dissociated  and  their  bands  cease  to  show.  All  of 
the  researches  concerning  spots  confirm  this  view  of  cooler 
temperatures,  the  low-temperature  furnace  lines  being 
strengthened  and  the  high  temperature  enhanced  lines  being 
weakened  in  spots. 

If  we  are  to  accept  the  explanation  of  the  Evershed  dis- 
placements that  they  are  a  Doppler  effect  which  differs  for 
lines  of  various  solar  intensities,  which  in  turn  depend  on  the 
depths  at  which  these  lines  originate,  then  we  are  forced  to 
the  conclusion  that  if  there  are  differences  in  level  between 
the  spots  and  the  photosphere  these  differences  cannot  be  as 
much  as  fifty  kilometers. 

The  motions  in  the  line  of  sight  of  the  gases  in  the  penum- 
bral  regions  of  sun-spots  can  be  measured  with  high  precision 
on  account  of  the  great  dispersion  employed  at  Mt.  Wilson. 
When  the  measures  of  the  Evershed  effect  from  the  individual 
Fe  lines  are  combined  into  averages,  and  the  spectral  lines  are 
then  arranged  in  various  manners  according  to  Rowland  in- 
tensities, heights  from  the  flash  spectrum,  excitation  poten- 
tials, and  spectral  regions,  conclusive  evidence  is  found  that 
the  size  of  the  Evershed  effect  does  depend  primarily  on  the 
heights.  On  the  assumption,  now  thoroughly  well  founded, 
that  a  spot  is  a  vortex  on  the  sun,  it  appears  abundantly  veri- 
fied that  the  indirect  determination  of  relative  heights  from 
the  Evershed  effect  tell  a  story  consistent  with  the  direct 
measurement  of  heights  from  the  flash  spectrum. 

By  referring  to  the  above  table,  these  interesting  facts  are 
observed :  ( i )  To  get  the  same  average  height  in  the  red  as 
to  the  violet  of  4900  A,  it  is  necessary  to  add  two  units  of  in- 
tensity to  the  right  in  the  table,  or  2  units  of  greater  strength 
in  the  red.  ( 2 )  In  quite  similar  fashion,  to  reach  the  same  ex- 


ECLIPSES  OF  THE  SUN 

citation  potential  in  the  red  as  in  the  violet,  it  is  necessary  to 
go  to  lines  of  2  Rowland  units  of  greater  intensity  in  red  than 
in  violet.  In  addition  (3)  St.  John  has  found  that  to  get  the 
same  measured  value  of  the  Evershed  effect  in  the  red  as  in 
the  violet,  it  is  necessary  to  go  to  2  units  of  greater  intensity 
in  the  red. 

From  the  calibration  of  Rowland's  intensity  scale,  it  was 
found  that  the  number  of  atoms  involved  in  the  formation  of 
a  line  of  Rowland  intensity  n  in  the  violet  is  approximately 
the  same  as  are  engaged  in  the  production  of  a  line  of  Row- 
land intensity  n  -f  2  in  the  red.  Hence  we  see  that  a  spectral 
line  produced  by  a  given  number  of  atoms  has  the  same  Ever- 
shed  effect,  the  same  height  in  the  flash  spectrum  and  the 
same  average  excitation  potential  throughout  the  spectrum. 
It  is  evident  that  there  is  one  underlying  cause,  and  one  only, 
needed  to  explain  these  interesting  correlations,  namely,  the 
number  of  atoms  involved.  But  how  has  this  information  so 
vitally  necessary  for  explaining  many  of  the  problems  con- 
nected with  solar  radiation  been  determined? 

After  arranging  the  lines  of  different  spectra  in  multiple! 
groups,  it  was  found  by  a  number  of  investigators 1  working 
independently,  and  almost  simultaneously,  that  simple  for- 
mulae give  the  relative  intensities  of  lines  belonging  to  a  given 
multiplet.  The  formulae  are  based  on  the  correspondence 
principle,  and  give  the  transition  probabilities  as  a  function 
of  the  quantum  numbers.  These  probabilities  when  multi- 
plied by  p4,  v  being  the  frequency,  are  proportional  to  the  en- 
ergy emitted  in  the  separate  lines.  Measures  show  that  these 
formulae  are  only  approximate,  but  on  the  whole  they  repre- 
sent the  intensities  with  fair  degree  of  accuracy.  According 
to  Ornstein,2  the  quantities  given  by  the  formulae  represent 
the  relative  numbers  of "  fictitious  resonators,"  or  the  relative 
numbers  of  atoms  involved  in  the  production  of  the  lines  of 
the  multiplets. 

Let  us  first  see,  therefore,  how  the  simple  rules  provided 
by  the  formulae  can  represent  different  arbitrary  estimates  of 

1  Kronig,  Zeits.  fur  Physik,  31,  885,  1925;   Sommerfeld  and  Honl,  Sitz.  der 
Preuss,  Akad.  der  Wiss.,  p.  141,  1925;  Russell,  Proc.  Nat.  A  cad.  of  Sciences,  u,  314, 
1925. 

2  Zeits.  fur  Physik,  40,  412,  1926. 


HEIGHTS  AND  DISTRIBUTIONS  OF  VAPORS     345 

intensities.  Take  first  the  estimates  by  A.  S.  King,  many 
thousands  in  numbers  of  the  spectra  of  the  lines  in  the  labora- 
tory. Russell  (loc.  cit.)  has  found  that  King's  scale,  which 
was  intended  to  represent  the  actual  intensities  of  the  lines,  is 
a  remarkably  homogeneous  scale.  These  Mt.  Wilson  inten- 
sities in  fact  are  very  nearly  proportional  to  the  square  roots 
of  the  relative  numbers  of  atoms  predicted  by  the  theory  of 
multiplets.  "  The  agreement  is  so  close  that  these  estimates, 
especially  when  averages  for  several  multiplets  are  available, 
are  clearly  almost  as  valuable  as  actual  measures,  when  once 
the  significance  of  the  empirical  scale  has  been  found." 

Another  comparison  of  even  greater  importance  is  the  cali- 
bration of  Rowland's  scale  of  intensities  for  the  solar  spec- 
trum. Russell,  Adams  and  Miss  Moore  by  means  of  the 
intensities  of  1288  lines  grouped  in  288  multiplets  have  inves- 
tigated l  Rowland's  scale  in  the  solar  spectrum.  The  relative 
intensities  in  multiplets,  based  on  the  correspondence  princi- 
ple, can  be  assumed  to  be  proportional  to  the  numbers  of  ac- 
tive atoms  producing  the  various  lines.  These  results  have 
most  important  consequences. 

By  the  observed  contours  of  a  number  of  typical  resonance 
lines  of  different  elements,  Unsold  has  determined J  the  num- 
ber of  atoms  above  i  sq.  cm  of  the  sun's  surface  which  are 
concerned  in  the  production  of  the  lines.  Although  many 
theoretical  and  practical  difficulties  have  still  to  be  sur- 
mounted and  much  more  observational  work  must  be  accom- 
plished on  the  contours  of  lines,  nevertheless  we  have  as  a 
result  what  is  perhaps  the  most  important  astrophysical  pub- 
lication in  recent  years.  This  comes  from  the  skillful  hands 
of  Russell J  entitled  "  On  the  Composition  of  the  Sun's  At- 
mosphere." Unfortunately  space  will  not  permit  a  detailed 
discussion  to  be  given  here. 

One  very  important  result  was  the  derivation  of  the  energy 
of  binding  of  an  electron  in  different  quantum  states  by  neu- 
tral and  ionized  atoms.  These  were  given  for  various  ele- 
ments in  tabular  form  together  with  tables  of  ionization  po- 
tentials. These  principles  were  first  applied  to  enhanced 

1  Astrophysical  Journal,  68 ,  i,  1926.  :*  Astrophysical  Journal,  70,  u,  1929. 

-  Zeits.  fur  Physik,  46,  765,  1928. 


346  ECLIPSES  OF  THE  SUN 

lines.  Russell  finds  that  thirteen  elements  have  their  most 
persistent  lines  in  the  region  of  the  spectrum  accessible  to 
photographic  observation.  These  elements  are  Bey  Ca,  Sc, 
Ti,  V,  Sr,  Y,  Zry  Cb,  Ba,  La,  Hf,  and  Ra;  all  but  the  last, 
radium,  have  enhanced  lines  in  the  sun.  Other  important  ele- 
ments like  Cr,  Mn,  Fe,  Co,  Ni  and  Mo  have  their  most  persist- 
ent lines  in  the  inaccessible  region  in  the  sun  to  the  violet  of 
wave-length  3000,  but  other  multiplets  arising  from  low 
energy-levels  are  accessible  for  investigation.  Fortunately 
with  few  exceptions  enhanced  lines  are  found  in  the  solar 
spectrum  for  all  those  elements  which  have  lines  of  low  ex- 
citation potential  in  the  accessible  region  of  the  spectrum. 
For  reasons  stated  in  foregoing  chapters,  these  elements  have 
their  lines  strengthened  in  the  chromosphere  and  hence  are 
prominent  in  the  flash  spectrum. 

On  the  contrary,  the  principal  factor  which  is  unfavorable 
to  the  appearance  of  a  spectral  line  in  the  sun  is  a  high  excita- 
tion potential.  As  a  matter  of  fact,  there  are  comparatively 
few  lines  in  the  solar  spectrum  with  excitation  potential 
greater  than  5  volts,  the  only  lines  of  great  strength  being  due 
to  hydrogen.  (In  the  flash  spectrum,  however,  is  found  the 
He 4~  line  at  4686  A,  with  excitation  potential  of  48  volts.) 

Hence  the  excitation  potentials  for  the  strongest  lines  in 
the  visible  part  of  the  spectrum  give  the  means  of  explaining 
the  presence  or  absence  of  lines  of  different  elements  in  the 
sun.  If  the  most  persistent  lines  are  not  observable  in  the 
solar  spectrum,  as  with  the  element  Mg,  the  method  can  give 
only  a  minimum  value  of  the  abundance. 

These  theoretical  considerations  depend  on  a  knowledge  of 
temperature  and  pressure  in  the  sun.  According  to  Edding- 
ton/  the  temperature  of  the  reversing  layer  doubtless  is  great- 
est closest  to  the  photosphere  and  may  be  assumed  to  be 
5730°  absolute.  Russell  finds  a  pressure  of  3  x  10  f;  atmos- 
pheres, in  good  agreement  with  the  conclusions  of  other 
investigators. 

The  total  quantity  of  the  metallic  elements  in  the  solar  at- 
mosphere seems  to  be  known  with  some  accuracy.  The  prob- 
lem of  the  non-metals  is  more  difficult,  mainly  on  account  of 

1  The  Internal  Constitution  of  the  Stars,  p.  332,  1926. 


HEIGHTS  AND  DISTRIBUTIONS  OF  VAPORS     347 

less  complete  observational  material.  The  metals  provide  all 
but  one-half  of  one  per  cent  of  all  the  ions  and  electrons.  If 
the  correction  for  departure  from  thermodynamic  equilibrium 
should  be  disregarded,  the  calculated  abundance  of  hydrogen 
would  be  increased  thirty-fold. 

Assuming  the  abundance  of  hydrogen  found  by  Menzel 
from  the  flash  spectrum  (and  which  the  present  writer  re- 
gards to  be  subject  to  large  systematic  errors),  then  Russell 
finds  the  probable  constitution  of  the  sun's  atmosphere  to  be: 

PROBABLE  CONSTITUTION  OF  THE  SUN'S  ATMOSPHERE 
Element  By  Volume  By  Weight 

Hydrogen 60  parts  60 

Helium 2?  8? 

Oxygen 2  32 

Metals i  32 

Free  Electrons 0.8  o 

Total 65.8  132 

Tables  are  given  for  the  relative  abundance  of  fifty-six 
elements  and  six  compounds.  Six  of  the  metallic  elements, 
Na,  Mg,  Si,  K,  Ca  and  Fe  contribute  95  per  cent  of  the  total 
mass  of  all  the  metals. 

Comparisons  made  with  Miss  Payne  *  show  an  excellent 
agreement  in  abundances  for  eighteen  of  the  most  important 
elements,  except  hydrogen  and  also  the  element  K  (prob- 
ably because  Miss  Payne's  value  for  the  latter  depends  on 
only  two  lines).  Comparisons  also  made  with  the  abun- 
dance of  elements  in  the  outer  ten  miles  of  the  earth's  crust, 
including  the  ocean  and  atmosphere,  and  in  stony  meteorites, 
show  a  good  agreement  throughout  (better  with  meteorites 
than  with  sun),  except  again  hydrogen.  The  difference  for 
H  is  enormous,  part  of  which  is  probably  real  and  part  is 
due  to  the  uncertainties  of  the  calculation. 

Russell  calls  attention  to  three  puzzles  still  outstanding: 
( i )  the  calculated  abundance  of  hydrogen  in  the  sun's  atmos- 
phere is  almost  incredibly  great;  (2)  the  electron  pressures 
calculated  from  the  degree  of  ionization  and  from  the  num- 
bers of  metallic  atoms  and  ions  are  discordant;  and  (3)  the 

1  Harvard  Observatory  Bulletin,  No.  835,  1926. 


348 


ECLIPSES  OF  THE  SUN 


calculated  rate  of  increase  of  density  with  depth  in  the  revers- 
ing layer  is  much  more  rapid  than  indicated  by  observations 
of  the  flash  spectrum. 

With  the  important  problems  involving  numbers  of  atoms 
so  clearly  stated  by  Russell  and  many  other  investigators,  we 
are  now  in  a  position  to  take  up  anew  the  question  of  what 
may  be  expected  within  a  multiplet  of  any  element. 

THE  Fe  MULTIPLEX  a5F  — y5!)0 


Fi 

Fi 

I1  A 

K- 

Fi 

(   3»20 

3887 

3940 

1         25 

cS 

4 

I>< 

l(5OO 

800 

500 

000 

005 

01  6 

O1O2 

0082 

0070 

3825 

3878 

3017 

20 

10 

5 

n* 

\     1500 

1000 

600 

1   ooo 

oo.s 

013 

0008 

OOQO 

0074 

f    3834 

3872 

3<So8 

14 

Q 

5 

1>2 

\     I2OO 

I  OOO 

OOO 

OO2 

003 

013 

OOQ4 

OOQO 

0074 

3840 

3805 

10 

8 

DI 

I  OOO 

800 

°°3 

00=5 

OOQO 

0082 

Wave-length 

3840 

Intensity  in  sun 

7 

Do       .           ... 

•  Height  in  kilometers 

75° 

Evershed  effect  in  sun-spots 
Sun  minus  vacuum  arc  in  angstroms 

006 
0078 

In  important  articles,  St.  John  '  and  Burns  ~  have  discussed 
their  own  particular  points  of  view  regarding  the  observa- 
tional details  within  a  given  multiplet.  For  the  purpose  of 
illustrating  the  "  unit  nature  "  of  multiplets,  both  have  util- 
ized the  same  Fe  multiplet,  ar'F — y5D°,  with  excitation  po- 
tential 0.92  volts.  After  a  thorough  study  of  the  whole 
problem,  by  comparing  this  njultiplet  with  others,  and  then 

1  Mt.  Wilson  Contributions,  389,  300,  1930. 

2  Journal  of  the  Optical  Society  of  America,  20,  212,  1930. 


H 

w 

o 


o  o 

H  ON 

O  M 

w  . 

dl  fO 

M 

H 


Q 

Q 


£  J 


«  o 

o 

5!  b 

>  2 

3  2 

B  fe 


o 

PM 
O 


• 


P 
CO 

M 

w 

H 


I 


o  - 

W 

5)  cd 

CL  l-i 

3  1 

w  S 


B   5 

• 


£ 
*o 

OH 


HEIGHTS  AND  DISTRIBUTIONS  OF  VAPORS     349 

smoothing  out  accidental  errors  and  getting  rid  of  the  effect 
of  blends,  the  appended  table  is  adopted  by  the  author  as 
his  idea  of  this  composite,  or  ideal,  multiplet.  The  top  num- 
ber in  each  case  is  the  wave-length,  below  this  is  the  intensity, 
then  come  in  order  for  each  line,  the  height  in  kilometers 
from  the  flash  spectrum,  the  Evershed  effect  measured  in 
angstroms,  and  finally,  the  difference  in  wave-length  between 
the  value  in  the  sun  minus  that  of  the  vacuum  arc.  The  Ein- 
stein, or  relativity,  shift  to  the  red  amounts  to  0.0082  A,  or  2 1 
parts  in  ten-million  in  wave-length. 

In  this  multiplet,  the  numbers  of  "  fictitious  resonators  " 
effective  in  forming  the  different  spectral  lines  vary  greatly. 
According  to  Russell  ( loc.  cit.,  p.  326)  more  than  one  hundred 
times  as  many  atoms  are  active  in  producing  the  strongest 
line  of  the  multiplet  at  3820  A  as  go  to  form  the  weakest  line 
at  3940  A.  Hence,  within  a  single  multiplet  one  should  expect 
that  the  heights  found  directly  from  the  flash  spectrum,  or 
indirectly  from  sun-spots,  would  not  be  constant  but  would  be 
greatest  for  the  largest  numbers  of  atoms. 

St.  John  and  Burns  differ  radically  in  the  interpretation 
of  the  systematic  differences  between  the  observations  for 
lines  of  different  intensities  in  the  table.  The  former  takes 
the  ground  that  the  relativity  shift  to  the  red  in  the  sun  has 
been  confirmed,  while  the  latter  is  still  unconvinced.  They 
agree,  however,  that  the  difference  sun  minus  vacuum  arc 
should  be  a  constant.  Burns  finds  (as  shown  in  the  table) 
that  the  more  intense  lines  have  a  greater  red  displacement 
than  the  less  intense  lines  in  the  same  multiplet  but  this  is  di- 
rectly contrary  to  St.  John's  deductions.  Burns  voices  his 
opinion  in  the  following  words:  "  It  seems  paradoxical  to  as- 
sume as  a  possibility  that  a  very  strong  line  of  a  multiplet  can 
originate  at  a  high  solar  level,  where  St.  John  postulates  the 
descending  movement,  while  a  weak  line  of  the  same  mul- 
tiplet originates  only  at  a  lower  solar  level  in  the  ascending 
vapor. " 

The  present  writer  is  inclined  to  agree  with  St.  John  and 
therefore  to  disagree  with  Burns  in  the  question  of  the  rela- 
tivity shift  in  the  sun;  he  agrees  with  Burns  and  disagrees 
with  St.  John  as  to  the  unequal  displacement  for  different 


350  ECLIPSES  OF  THE  SUN 

lines  in  a  multiplet;  but  he  disagrees  with  both  St.  John 
and  Burns  in  the  interpretation  of  the  data  observed  within 
single  multiplets. 

Two  strong  multiplets  of  Fe  are  specially  valuable  for  pur- 
poses of  comparison,  namely ,  a  'D  —  z  D(),  with  average  ex- 
citation potential  0.07  volts,  and  ar'F  —  yr'D°,  with  excitation 
potential  0.92  (the  composite  multiplet  given  in  the  table). 
These  two  multiplets  have  the  same  maximum  Rowland  in- 
tensity of  20  in  each  case,  and  nearly  the  same  minimum 
intensity.  Moreover,  the  two  multiplets  cover  approxi- 
mately the  same  spectral  region,  from  3820  to  3940  A,  and 
hence  no  systematic  effects  depending  on  wave-length  need 
be  considered.  Although  the  Rowland  intensities  average 
the  same  in  the  two  multiplets,  there  are  pronounced  differ- 
ences in  the  other  observed  quantities.  The  multiplet  of 
low  excitation  potential  0.07  volts  compared  with  that  of  the 
higher  value  0.92  shows  the  following:  ( i )  greater  intensities 
in  the  flash  spectrum,  (2)  greater  heights  and  (3)  greater 
values  of  the  wave-length  difference  sun  minus  vacuum  arc. 
Each  multiplet  separately  for  ( i ) ,  ( 2 )  and  ( 3  )  show  higher 
values  on  the  main  diagonal  than  on  the  side  diagonals.  For 
a  height  of  2000  km  from  the  multiplet  with  excitation  po- 
tential 0.07,  the  difference  sun  minus  vacuum  arc  is  -f-  0-0135 
angstroms  which,  corrected  for  the  relative  shift  0.0082  A, 
gives  a  residual  difference  +  0.0053  angstroms  correspond- 
ing to  a  downward  motion  in  the  sun's  atmosphere  of  0.4  km 
per  second. 

It  must  be  emphasized  again  and  again  that  the  "  ef- 
fective height "  above  the  photosphere  at  which  a  line  in  the 
Fraunhofer  spectrum  is  formed,  the  heights  from  eclipse 
spectra  and  the  heights  from  the  Evershed  effect,  do  not  and 
cannot  refer  to  the  maximum  heights  to  which  atoms  are 
ejected  by  solar  activity.  All  three  heights  are  derived  from 
photographic  plates,  all  depend  on  the  effects  produced  by 
the  atoms  on  the  photograph.  As  already  stated  in  the  fore- 
going, the  atoms  must  be  in  sufficient  numbers,  or  have  a 
concentration  adequate  to  leave  a  trace  of  their  action  on  the 
photographic  plate.  It  is  not  impossible  that  under  average 
conditions  of  solar  activity,  all  Fe  atoms  no  matter  what  are 


HEIGHTS  AND  DISTRIBUTIONS  OF  VAPORS     351 

their  atomic  levels  or  whether  they  are  neutral  or  enhanced 
reach  about  the  same  maximum  heights  in  the  chromosphere. 
If  the  solar  activity  becomes  greater,  as  in  the  course  of 
the  sun-spot  cycle,  or  locally  as  in  prominences,  the  heights 
become  greater. 

The  conditions  under  which  atoms  in  the  chromosphere 
radiate  to  form  the  flash  spectrum  (or  absorb  radiation  in 
the  dark-line  spectrum)  are  becoming  well  understood. 
They  depend  on  temperature,  pressure  and  concentration  of 
atoms.  For  a  single  multiplet,  it  is  obviously  necessary  to 
assume  that  all  of  the  spectral  lines  are  formed  under  nearly 
identical  conditions  of  temperature  and  pressure  and  that  all 
the  atoms  reach  the  same  maximum  heights.  The  lines  dif- 
fer greatly  the  one  from  the  other  in  the  number  of  atoms  in- 
volved in  the  emission  (or  absorption)  of  radiation.  Let  us 
now  confine  our  attention  to  two  lines  from  a  multiplet,  the 
one  produced  by  one  hundred  times  as  many  atoms  as  the 
other.  Consider  for  each  line  a  cylinder  of  unit  cross  sec- 
tion. The  base  of  each  cylinder  we  shall  call  the  photosphere, 
each  axis  being  perpendicular  to  the  solar  surface.  The  top 
of  each  cylinder  is  at  an  equal,  but  at  a  very  great  distance 
above  the  photosphere.  The  atoms  are  most  concentrated 
near  the  photosphere.  Looking  down  into  the  axis  of  each 
cylinder,  that  is,  observing  a  spectral  line  at  the  center  of 
the  sun's  disk  under  the  ordinary  conditions  of  the  Fraun- 
hofer  spectrum,  it  is  possible  to  see  down  into  each  cylinder 
until  the  atoms  become  so  concentrated  that  they  virtually 
form  a  black  wall  through  which  vision  cannot  penetrate. 
This  black  wall  in  each  case  then  becomes  the  "  effective 
height  "  above  the  photosphere  at  which  each  spectral  line 
is  formed.  The  stronger  line  of  the  two,  the  one  involving 
the  greater  number  of  atoms,  has  its  effective  level,  or  takes 
its  origin,  at  a  greater  height  above  the  photosphere  than  the 
weaker  line  of  fewer  atoms.  The  above  expresses  in  non- 
technical language  what  various  authorities  for  a  number 
of  years  have  been  stating  in  technical  terms.  This  simple 
scheme  seems  adequate  to  explain  a  great  variety  of  solar 
phenomena.  In  particular,  it  seems  easy  to  understand  that 
within  a  single  multiplet  the  spectral  lines  take  their  origin 


352  ECLIPSES  OF  THE  SUN 

at  different  effective  heights,  the  strongest  lines  at  the  great- 
est heights  and  the  weakest  at  the  least  heights  above  the 
photosphere. 

Through  a  splendid  series  of  researches  carried  out  at 
Mt.  Wilson  and  elsewhere,  a  very  imposing  structure  has 
been  built  on  the  assumption  that  Fraunhofer  lines  do  in- 
deed take  their  origins  at  different  heights  above  the  photo- 
sphere. It  seems  entirely  unnecessary  to  overthrow  this 
beautiful  edifice  by  requiring  that  all  lines  in  a  multiplet,  no 
matter  how  much  they  differ  in  intensities,  must  all  be 
found  at  the  identically  same  heights.  It  seems  equally 
unnecessary  to  assume  that  all  lines  in  a  multiplet,  involving 
as  they  do  very  different  numbers  of  atoms,  must  exhibit 
the  same  difference  in  wave-length  between  sun  and  vacuum 
arc.  In  the  composite  multiplet,  ar'F  —  y  D(>,  the  intensities 
of  the  strongest  and  weakest  lines  of  the  multiplet  are  as 
25:4  or  6: i,  the  heights  are  approximately  as  3:1  and  the 
differences  sun  minus  vacuum  arc  are  as  i  \ :  i. 

From  his  discussion,  St.  John  finds  that  the  strong  solar 
lines  show  a  greater  red  shift  than  do  the  weaker  lines  except 
in  the  case  where  strong  and  weak  lines  occur  in  the  same 
multiplet,  and  then  the  red  shift  must  perforce  be  the  same 
for  all  lines  no  matter  how  much  they  differ  in  intensities! 
On  the  other  hand,  both  Burns  and  the  writer  find  the  greater 
shift  for  the  strong  lines  even  though  the  strong  and  weak 
lines  belong  to  the  same  multiplet.  Burns  attempts,  without 
much  success,  to  explain  the  greater  red  shift  of  the  strong 
lines  as  due  to  an  "  intensity  equation  "  partly  due  to  in- 
strumental causes.  Both  Burns  and  St.  John  find  fault  with 
the  heights  from  the  flash  spectrum  in  that  these  heights  are 
not  a  constant  for  all  of  the  spectral  lines  of  a  multiplet.  The 
problem  of  the  relativity  shift  in  the  solar  spectrum  will 
be  taken  up  in  greater  detail  in  Chapter  XXIII. 

The  theory  of  the  formation  of  Fraunhofer  lines  and  the 
problem  of  determining  the  relative  abundances  of  elements 
in  the  sun's  atmosphere  is  now  (1932)  receiving  much  at- 
tention by  astrophysicists.  The  puzzles  noted  by  Russell  on 
page  347  show  that  theory  and  observation  are  not  yet  in 
satisfactory  accord.  With  the  perfecting  of  the  registering 


HEIGHTS  AND  DISTRIBUTIONS  OF  VAPORS     353 

microphotometer  much  observational  work  has  been  done  on 
the  difficult  task  of  measuring  the  contours  of  spectral  lines  in 
the  stars  and  in  the  sun.  On  account  of  the  vastly  greater  dis- 
persion possible  with  the  sun,  it  is  evident  that  the  chief  ad- 
vances must  come  from  solar  measures.  The  purpose  of  these 
measures  is  to  find  the  number,  N,  of  atoms  involved,  par- 
ticularly in  the  formation  of  resonance  lines  of  different  ele- 
ments. In  addition  to  Unsold,  many  other  investigators  have 
engaged  in  the  problem.  Wooley '  employed  a  clever  method 
of  attack  by  measuring  the  widths  of  the  terrestrial  atmos- 
pheric lines  in  the  B-band  of  the  solar  spectrum.  With  the 
observations  made  at  different  times  during  the  day,  the 
known  altitudes  gave  a  ready  means  of  deriving  the  relative 
numbers  of  atoms  involved.  Wooley  found  a  variation  in 
the  widths  of  lines  proportional  to  N*  instead  of  N*,  as  ex- 
pected by  Unsold 's  theories.  He  then  came  to  the  conclusion 
that  the  difference  between  theory  and  observation  was 
caused  more  by  Unsold's  method  rather  than  by  incorrect 
calibration  by  Rowland.  H.  H.  Plaskett 2  measured  the  mag- 
nesium 6-triplet  in  the  sun's  spectrum.  He  discussed  his 
measures  by  means  of  the  following  assumptions:  (i)  pure 
absorption,  (2)  pure  scattering,  and  (3)  combined  absorp- 
tion and  scattering.  The  third  assumption  seems  best  to  fit 
both  theory  and  observation.  Pannekoek  and  Minnaert, 
together  and  separately,  have  made  valuable  contributions 
to  both  the  theoretical  and  observational  sides.  As  pointed 
out  in  earlier  chapters,  Milne  has  done  very  valuable  work 
on  the  theoretical  side.  It  is  evident  that  enormous  progress 
has  already  been  made  as  a  result  of  the  new  quantum  theory, 
but  the  goal  still  seems  far  distant. 

In  spite  of  the  imperfect  agreement  between  observation 
and  theory,  it  will  be  very  important  to  follow  Russell's 
methods  and  ascertain  the  abundances  of  elements  in  the 
sun's  atmosphere  from  the  flash  spectrum.  The  best  pho- 
tographs for  this  purpose  are  unquestionably  those  of  the 
Lick  Observatory  discussed  by  Menzel  and  the  concave 
grating  spectra  taken  by  Mitchell.  The  information  from 

1  Astro  physical  Journal,  73,  185,  194,  1931. 

2  Monthly  Notices,  R.  A.  S.,  91,  870,  1931. 


354  ECLIPSES  OF  THE  SUN 

the  latter  should  be  the  more  reliable,  for  the  reasons  that 
the  grating  spectra  were  photographed  almost  completely  by 
one  spectrograph  with  constant  dispersion,  while  the  Lick 
spectra  were  taken  with  several  spectrographs  and  varying 
dispersions,  with  both  fixed  and  moving  plates;  and  in  addi- 
tion they  covered  only  about  one-half  of  the  region  of  wave- 
lengths discussed  by  Mitchell. 

Both  Menzel  and  Mitchell  attempted  to  estimate  their 
intensities  on  the  Rowland  scale.  In  general,  each  agreed 
with  Rowland  and  with  each  other.  However,  in  the  fore- 
going chapters  the  large  systematic  differences  in  intensities 
between  the  chromospheric  intensities  and  those  of  Row- 
land have  been  emphasized.  These  differences  may  be 
briefly  summarized  as  follows:  (i)  The  hydrogen  and  he- 
lium lines  are  much  stronger  in  the  chromosphere.  (2)  The 
enhanced  or  ionized  lines  are  likewise  much  stronger.  The 
great  increase  in  strength  has  been  explained  by  the  fact 
that  the  intensified  lines  are  high-level  lines.  (3)  With  the 
neutral  lines  in  the  spectra  of  the  metals,  similar  systematic 
differences  between  the  chromosphere  and  Rowland  are 
noted.  It  was  shown  that  for  any  element,  the  strongest  lines, 
or  those  of  low  excitation  potential,  are  stronger  in  the  chro- 
mosphere than  in  Rowland;  those  of  medium  excitation  po- 
tential have  about  equal  intensities  in  the  two  spectra,  while 
the  lines  of  highest  excitation  potential  are  weaker  in  the 
chromosphere. 

The  reasons  for  these  systematic  differences  for  the  neu- 
tral lines  find  a  ready  explanation;  namely,  lines  of  low  ex- 
citation potential  involve  more  atoms  than  those  of  the  same 
element  of  high  excitation  potential.  The  greater  number 
of  atoms  involved  permits  these  lines  to  be  detected  to 
greater  heights  in  the  flash  spectrum.  Apparently,  therefore, 
all  high-level  lines  whether  from  the  neutral  or  the  ionized 
atom  are  stronger  in  the  flash  spectrum  than  in  Rowland, 
while  on  the  contrary,  those  of  low  levels  are  weaker  than 
in  Rowland. 

Remembering,  therefore,  that  Russell's  determination  of 
the  abundances  in  the  sun's  atmosphere  depends  primarily 
on  the  ability  to  calibrate  the  Rowland  scale,  it  might  be 


HEIGHTS  AND  DISTRIBUTIONS  OF  VAPORS     355 

thought  that  on  account  of  the  great  differences  just  noted 
between  the  Rowland  and  the  chromospheric  intensities, 
there  might  be  systematic  differences  in  the  amounts  of  the 
elements  found  from  the  Fraunhofer  spectrum  and  from  the 
flash  spectrum.  In  particular  it  might  be  thought  that  H 
and  He,  on  account  of  their  great  strengths,  would  be  rela- 
tively much  more  abundant  from  the  chromospheric  investi- 
gations. Manifestly,  it  is  necessary  also  to  calibrate  the 
estimated  intensities  in  the  flash  spectrum.  The  reliability 
of  the  final  results  will  depend  mainly  on  how  accurately 
this  calibration  can  be  carried  out. 

As  Russell  has  pointed  out,  the  information  for  each  of 
the  elements  found  in  the  sun  depends  chiefly  on  the  strong 
lines,  those  of  low  excitation  potential.  In  spite  of  the  much 
greater  dispersion  in  the  Rowland  spectrum  than  is  avail- 
able in  the  flash  spectrum,  the  latter  has  almost  as  complete 
information  regarding  the  strong  lines.  Hence  the  results  for 
the  chromosphere  from  eclipse  spectra  should  have  almost 
the  same  accuracy  as  those  of  Russell. 

Two  separate  determinations  have  been  made:  one  from 
the  Lick  photographs  by  Menzel,  the  other l  by  Mitchell  and 
Miss  Williams.  The  results  show  that  within  the  limit  of 
errors  there  appears  to  be  little  difference  between  the  rela- 
tive abundances  derived  from  the  Fraunhofer  spectrum  and 
those  obtained  from  the  flash  spectrum.  In  other  words,  the 
two  determinations  from  the  chromosphere  differ  from  each 
other  about  as  much  as  either  differs  from  Russell's  deter- 
mination. This  is  true  for  all  of  the  elements  investigated 
with  the  exception  of  H  and  He.  For  obvious  reasons,  the 
information  from  the  chromospheric  spectra  for  these  two 
elements  should  be  more  reliable  than  from  the  Fraunhofer 
spectrum. 

With  the  results  now  available,  it  seems  probable  that 
hydrogen  is  actually  more  abundant  in  the  chromosphere, 
but  whether  the  factor  is  ten,  or  ten  thousand  times,  we  are 
not  now  sure.  In  fact,  the  lightest  elements,  hydrogen  and 
helium,  still  remain  puzzles.  In  spite  of  the  very  great  num- 
ber of  attempts  that  have  been  made  to  predict  from  theory 

1  Publ.  Leander  McCormick  Obsy.,  5,  Pt.  6, 


356  ECLIPSES  OF  THE  SUN 

the  intensities  in  the  hydrogen  spectrum,  no  very  great 
success  has  yet  been  attained. 

Additional  information  obtained  from  eclipse  spectra  is 
the  distribution  of  atoms  at  different  heights,  or  in  other 
words,  the  density  gradients  in  the  chromosphere.  Panne- 
koek  and  Minnaert,  from  their  1927  spectra,  found  density 
gradients  up  to  2000  km  for  the  emitting  atoms  of  H  and  He. 
Their  measures  showed  that  the  density  of  hydrogen  de- 
creased much  less  rapidly  than  would  be  expected  in  an  iso- 
thermal atmosphere  under  equilibrium  conditions.  The 
writer  and  Miss  Williams  (loc.  tit.}  have  derived  density 
gradients  for  the  emitting  atoms  of  neutral  Fe  and  for  the 
ionized  elements  Ti,  Fe  and  Cr  between  the  heights  of  300 
and  2500  km.  Menzel  (loc.  tit.)  has  derived  a  density  law 
up  to  2500  km  by  combining  the  measures  from  different 
elements. 

To  derive  the  density  gradients,  it  is  necessary  to  know 
the  heights  as  accurately  as  possible.  It  is  also  necessary 
that  the  atomic  origin  of  the  lines  be  known  and  that  the 
lines  arise  from  multiplets  in  which  the  relative  intensities 
of  the  lines  can  be  predicted  by  the  theoretical  intensity  for- 
mulae. The  inter-system  lines  are  of  no  use  for  the  purpose 
for  the  reason  that  no  intensity  formulae  are  known  for 
them.  Further,  the  lines  of  the  Balmer  series  of  hydrogen 
cannot  be  used  because  no  two  lines  belong  to  the  same  multi- 
plet.  In  the  case  of  an  element  like  Fe,  there  is  a  great 
amount  of  material  available.  Every  multiplet  in  which  two 
or  more  lines  appear  unblended  in  the  flash  spectrum  fur- 
nishes a  determination  of  the  intensity  gradient  at  different 
heights  above  the  photosphere. 

Earlier  in  this  chapter,  the  multiplet  a5D  —  yr'D°  has  been 
considered.  The  intensity  formulae  show  that  the  relative 
numbers  of  atoms  involved  per  unit  volume  are  nearly  pro- 
portional to  the  squares  of  the  intensities.  For  purposes 
of  illustration,  we  shall  group  together  lines  of  equal 
heights  in  the  multiplet  and  round  off  the  predicted  number 
of  atoms.  From  the  12  lines  of  the  multiplet  we  have  the 
following: 


HEIGHTS  AND  DISTRIBUTIONS  OF  VAPORS     357 


Line 

a 

b 

c 

d 

e 

/ 

Relative  number  of  atoms 

45° 

300 

200 

100 

60 

5 

Height  in  kilometers.    .    . 

1600 

1500 

1  200 

1000 

800 

600 

With  photographs  of  the  flash  spectrum  taken  without  slit, 
measures  of  the  chords  to  the  tips  of  the  cusps  give  the 
heights.  As  explained  in  the  foregoing,  a  detectable  blacken- 
ing of  the  photographic  plate  occurs  when  there  is  a  given 
number  of  emitting  atoms  along  the  line  of  sight,  a  state- 
ment which  is  true  only  within  a  limited  region  of  wave- 
lengths. Hence  the  number  of  atoms  in  line  b,  above  the 
i50o-km  level  from  the  photosphere,  is  exactly  equal  to  the 
number  in  line  d  above  the  looo-km  level,  or  in  line  /  above 
the  6oo-km  level.  Evidently  in  this  multiplet,  i^  times  as 
many  atoms  are  detected  by  the  photographic  plate  in  the 
act  of  emitting  radiation  at  the  i20o-km  level  as  at  the  1500- 
km  level.  The  number  of  atoms  per  unit  volume  being  i^ 
times,  the  density  of  concentration  is  therefore  50  per  cent 
greater.  Compared  with  the  density  at  the  isoo-km  level, 
the  density  at  the  i2oo-km  level  is  i^  times,  at  the  looo-km 
level  it  is  3  times,  at  Soo-km  it  is  5  times,  and  at  6oo-km  the 
density  is  60  times.  Hence,  it  is  evident  that  from  this  one 
multiplet,  the  densities  of  distribution  of  neutral  Fe  atoms  can 
be  obtained  at  different  heights  above  the  photosphere. 
Other  multiplets  give  similar  information,  and  hence  the 
density  gradients  may  be  ascertained  by  taking  the  averages 
for  all  the  multiplets  employed.  Averages  being  taken  for 
different  heights,  it  is  manifest  that  the  most  reliable  results 
are  derived  from  those  spectra  for  which  the  heights  are 
most  accurately  known.  For  any  element  like  Fe  where 
many  multiplets  are  available,  the  material  may  be  divided 
into  different  groups  of  low,  medium  and  high  excitation 
potential.  When  the  number  of  multiplets  are  numerous 
enough,  this  same  process  may  be  carried  out  with  different 
neutral  and  ionized  elements.  When  due  account  has  been 
taken  of  these  matters,  it  is  possible  to  derive  an  empirical 
density  law  for  a  number  of  different  elements. 


358  ECLIPSES  OF  THE  SUN 

In  MenzePs  discussion,  all  elements  were  grouped  together 
while  Mitchell  and  Miss  Williams  discussed  the  elements 
separately.  The  results  from  the  two  discussions  agree,  and 
they  show  that  the  density  gradients  for  the  various  elements 
are  not  very  different.  The  density  gradients  for  neutral 
Fe  and  for  ionized  Ti  are  not  very  much  greater  than  for 
hydrogen;  and  all  values  are  much  less  than  would  be  ex- 
pected in  an  isothermal  atmosphere. 

The  difference  found  in  the  distribution  of  the  atoms  of  Fe 
of  low  excitation  potential,  as  compared  with  that  of  the 
atoms  of  higher  excitation  potential,  might  be  ascribed  to 
a  large  temperature  gradient  in  the  chromosphere  were  it 
not  for  the  fact  that  the  Ti-\-  data  shows  no  such  effect. 
The  same  effect  is  undoubtedly  present  in  the  Fc  data  at 
much  lower  levels  —  in  the  regions  between  the  effective 
levels  of  the  reversing  layer  and  the  chromosphere.  Menzel 
ascribes  this  to  a  temperature  gradient,  but  in  view  of  the 
fact  that  the  Ti  +  data  shows  no  such  effect  in  this  region, 
even  as  it  showed  no  effect  at  greater  heights,  it  is  unsatisfac- 
tory to  assume  an  explanation  which  brings  the  Fe  data  into 
step  but  at  the  same  time  throws  the  Ti  +  data  out  of  step. 

But  why  are  the  density  gradients  so  much  smaller  than 
expected?  What  is  the  supporting  mechanism?  Many  theo- 
ries have  been  proposed.  These  have  been  examined  in  de- 
tail by  Menzel.  Here  it  will  be  possible  to  touch  on  them 
very  briefly,  on  account  of  limitations  of  space  and  also  for 
the  reason  that  none  of  the  theories  satisfactorily  explain  the 
observed  facts.  Milne  (loc.  cit.)  has  a  beautiful  theory  in 
regard  to  ionized  calcium.  The  atoms  fall  toward  the  sun 
under  the  influence  of  gravity  until  a  quantum  of  energy  of 
the  frequency  of  the  H  or  K  line  is  absorbed.  The  momen- 
tum associated  with  the  light-quantum  tosses  the  atom  high 
into  the  chromosphere  again,  after  which  once  again  it 
begins  to  fall.  This  process  is  repeated  indefinitely.  This 
theory  may  be  applied  to  the  other  alkaline  earths,  Mg,  Sr 
and  Ba,  but  it  is  not  applicable  to  any  of  the  other  elements ; 
some  other  force  must  support  them.  As  Rosseland  !  was 
the  first  to  suggest,  there  is  a  great  turbulence  in  the  sun's 

1  Monthly  Notices,  R.  A.  S.,  88,  377,  1928. 


HEIGHTS   AND  DISTRIBUTIONS  OF  VAPORS      359 

atmosphere  in  virtue  of  which  the  heavy  elements  will  be 
tossed  to  great  heights  in  the  chromosphere  along  with  the 
light  elements.  This  would  account  for  the  fact  that  the 
relative  abundances  found  in  the  reversing  layer,  that  is,  in 
the  lower  and  denser  layers  of  the  chromosphere,  are  prac- 
tically the  same  as  found  in  the  higher  reaches  of  the  chro- 
mosphere from  eclipse  spectra.  Turbulence  would  also 
account  for  the  similar  density  gradients  of  dissimilar 
elements. 

Gurney's  theory  1  is  most  interesting,  especially  as  it  may 
give  a  solution  for  the  helium  puzzle.  Since  there  are  so 
many  Ca  -f-  atoms  in  the  high  chromosphere,  a  great  number 
of  them  will  become  doubly  ionized.  But  the  doubly  ionized 
Ca  ion  is  incapable  of  supporting  itself  by  absorbing  light- 
quanta,  for  the  reason  that  its  ultimate  lines  lie  in  a  spectral 
region  where  the  number  of  light-quanta  emitted  by  the  sun 
are  relatively  few.  Hence  the  Ca  +  +  ion  will  drop  back  to 
the  photosphere  and  in  so  doing  it  will  attain  a  tremendous 
kinetic  energy,  enough  to  excite  the  visible  helium  lines  if  it 
should  collide  with  a  He  atom,  or  even  enough  to  ionize  the 
He  and  excite  the  interesting  line  found  in  the  flash  spectrum 
at  wave-length  4686. 

1  Ibid.,  89,  49,  1928. 


T 


CHAPTER  XX 

THE  CORONA 

corona  still  remains  exclusively  an  eclipse  phe- 
nomenon. In  spite  of  the  amazing  achievements 
of  modern  science  which  at  times  seems  to  be  able 
almost  to  accomplish  the  impossible,  no  success  has  attended 
the  efforts  made  to  observe  the  corona  outside  of  an  eclipse. 

On  account  of  the  dramatic  character  of  the  phenomenon 
and  of  the  great  interest  in  eclipses  felt  by  both  the  astrono- 
mer and  the  general  public,  each  eclipse  as  it  comes  is  en- 
thusiastically observed ;  but  the  truth  must  be  told  that  suc- 
cess commensurate  with  the  labor  involved  is  not  always 
forthcoming.  "  The  problems  of  the  corona  are  many,  and 
few  of  them  can  be  said  to  have  approached  solution.  Im- 
portant facts  concerning  it  have  been  established,  but  these 
facts  are  more  or  less  isolated,  and  in  general  their  relations 
to  each  other  are  unknown.  The  paucity  of  results  obtained 
thus  far  is  due  primarily  to  the  unique  condition  that  the 
most  assiduous  of  eclipse  observers  can  scarcely  hope  for 
more  than  an  hour  of  totality  with  clear  skies,  in  his  entire 
lifetime/'  In  this  and  the  following  chapter  we  shall  attempt 
to  show  how  the  amazing  progress  of  recent  years  in  every 
branch  of  solar  research  has  affected  the  solution  of  coronal 
problems. 

Concerning  the  total  eclipse  of  March  29,  1652,  seen  in 
Ireland,  Dr.  Wyberd  writes,  "  The  moon  suddenly  threw  it- 
self within  the  solar  disk  with  such  agility  that  it  seemed 
to  go  round  like  an  upper  mill-stone.  The  sun  then  appeared 
around  the  limb,  affording  a  pleasant  and  remarkable  spec- 
tacle of  rotation."  There  seemed  to  be  a  widespread  notion 
among  the  early  observers  of  eclipses  that  during  totality 
very  rapid  motions  took  place  within  the  corona  and 
that  the  corona  was  some  sort  of  a  modern  fireworks 

360 


THE  CORONA  361 

display  with  brilliant  scintillations  and  sudden  changes.  In 
fact,  the  early  notion  seems  not  to  have  entirely  vanished 
in  the  enlightened  and  scientific  age  of  the  twentieth 
century. 

No  eclipse  expedition  worthy  of  the  name  will  be  fully 
equipped  unless  it  has  as  part  of  its  program  the  securing 
of  large  scale  photographs  of  the  corona.  Astronomy  of  the 
future  needs,  as  it  has  in  the  past,  to  secure  good  photographs 
of  every  possible  eclipse.  The  Lick  Observatory  has  the 
most  complete  series  in  existence,  beginning  with  the  eclipse 
of  1893.  The  Lick  photographs  have  always  been  secured  by 
pointing  the  camera  directly  at  the  sun,  the  method  devised 
by  Schaeberle,  and  a  uniform  focal  length  of  forty  feet  has 
been  employed.  This  permanent  record  of  the  past  is  always 
available  for  purposes  of  comparison. 

The  5-inch  aperture,  4O-foot  focus  represents  a  ratio  of 
aperture  to  focal  length  of  1:96.  The  commercial  photog- 
rapher would  look  aghast  if  he  were  compelled  to  work  with 
such  a  slow  camera.  For  investigating  the  details  of  the 
inner  corona,  a  large  scale  image  is  necessary,  rendered  pos- 
sible by  great  focal  length.  Up  to  the  present,  the  largest 
scale  has  been  secured  at  the  eclipse  of  1900  through  the  use 
of  a  horizontal  camera  of  12  inches  aperture  and  135  feet 
focal  length,  a  ratio  a:/  =1:135.  The  brightness  of  the 
inner  corona  permits  the  employment  of  short  exposures  even 
with  the  small  ratio  of  aperture  to  focal  length  necessitated 
by  cameras  of  great  focal  length. 

Two  methods  of  mounting  cameras  of  great  length  are 
available,  either  that  of  pointing  the  objective  directly  at  the 
sun,  or  using  the  horizontal  telescope  with  coelostat  mirror. 
The  former  has  distinct  advantages  over  the  latter,  but  the 
erection  becomes  increasingly  difficult  as  focal  lengths  are 
augmented.  Miller  of  Swarthmore  College  has  been  very 
successful  with  focal  lengths  of  63  feet.  For  such  a  mount- 
ing, a  double  tower  is  necessary,  an  inner  one  to  carry  the 
objective,  and  an  outer  tower  to  protect  the  whole  from  jars 
caused  by  the  wind.  With  this  type  of  mounting  the  plate  is 
moved  by  a  clock  to  counteract  the  diurnal  motion.  With 
the  Swarthmore  tower-telescope  on  Niuafoou  Island,  at  the 


362  ECLIPSES  OF  THE  SUN 

most  recent  eclipse,  photographs  with  superb  definition  were 
secured  by  Marriott. 

The  horizontal  telescope  is  much  easier  to  construct  and 
erect  in  the  field.  The  tube  may  be  made  of  heavy  paper  and 
the  plate  holders  carried  by  a  heavy  framework  surrounded 
by  a  dark  room.  Care  should  be  taken  that  the  tube  is  not 
too  near  the  ground  and  that  the  tube  is  protected  from  the 
direct  rays  of  the  sun  by  a  canvas  or  paper  shelter.  The 
coelostat  is  the  best  form  of  mounting  for  the  plane  mirror. 
Compared  with  direct  mounting,  as  was  effectively  shown  at 
the  eclipse  of  1919,  the  horizontal  telescope  has  the  great 
disadvantage  that  the  mirror  is  sensitive  to  changes  of  tem- 
perature and  it  may  alter  its  shape  and  become  warped  on 
eclipse  day  when  exposed  to  the  sun's  rays.  Ordinarily,  focus 
is  obtained  for  eclipse  cameras  by  star  trails.  If  possible  a 
warm  night,  differing  as  little  as  possible  from  the  expected 
temperature  at  eclipse  time,  should  be  utilized.  It  is  impera- 
tively necessary  to  keep  the  mirror  protected  from  the  sun's 
rays  on  the  day  of  the  eclipse  until  a  very  few  minutes  before 
totality.  One  should  avoid  the  horizontal  telescope  if  pos- 
sible, mainly  on  account  of  the  uncertainties  connected  with 
the  mirror  and  its  lack  of  permanent  figure.  At  the  Niuafoou 
Island  eclipse,  good  photographs  were  secured  with  the  65- 
foot  horizontal  telescope  belonging  to  the  U.  S.  Naval  Ob- 
servatory. The  lens  is  as  good  a  one  as  the  Swarthmore  lens 
of  nearly  equal  focal  length  used  in  the  tower-telescope  and, 
moreover,  Marriott  cared  for  the  details  of  both  instruments 
and  the  development  of  the  plates.  The  definition  of  the 
tower  plates,  however,  was  better  than  those  taken  with 
the  horizontal  telescope.  For  the  purpose  of  portraying  the 
structural  details  of  the  corona  it  is  evident  that  the  greater 
the  focal  length  utilized  the  more  valuable  will  be  the  results. 
Cameras  of  medium  and  small  size  have  not  been  superseded 
by  the  large  instruments  and  still  have  useful  functions  at 
eclipses.  Larger  ratios  of  aperture  to  focal  length  are  possi- 
ble than  with  those  of  the  largest  size.  Such  cameras  are 
more  rapid  and,  consequently,  are  useful  in  securing  the  faint 
extensions  of  the  outlying  corona.  They  are  especially  valu- 
able in  photometric  observations  which  will  be  taken  up  in 


THE  CORONA  363 

detail  below.  Every  precaution  should  be  taken  to  reduce 
to  a  minimum  the  effects  of  halation  and  reflections  from  the 
glass-side  of  the  plates.  All  plates  used  for  photographing 
the  corona  should  be  "  backed  "  with  some  absorbing  material 
on  the  glass  side.  Some  eclipse  observers  have  had  excellent 
success  from  using  double-coated  or  triple-coated  plates. 

In  attempting  to  photograph  the  corona  on  small  or  large 
scale,  it  goes  without  saying  that  the  greatest  care  should 
be  exercised  to  secure  the  most  perfect  definition,  and  also 
that  the  photographic  manipulation  should  be  conducted 
so  as  to  bring  out  of  the  plates  as  much  of  the  wealth  of 
detail  as  possible.  Development  may  be  carried  out  in  such 
a  way  as  to  attain  very  different  photographic  effects.  If 
contrast  is  needed,  as  in  spectrum  work,  a  "  hard  "  develop- 
ment is  required.  There  are  many  developers  suited  for  this 
sort  of  work  with  which  anyone  doing  spectrum  work  is  en- 
tirely familiar.  For  developing  the  corona,  however,  where 
it  is  necessary  to  bring  out  as  many  of  the  fine  details  as  pos- 
sible, and  where  an  attempt  should  be  made  to  minimize  the 
contrast  between  the  bright  inner  corona  and  the  faint  outer 
corona,  an  entirely  different  kind  of  developer  and  develop- 
ment is  necessary.  Old-fashioned  "  pyro  "  is  probably  the 
best  form  of  developer  to  use  for  this  purpose,  and  one  would 
do  best  to  start  with  a  very  weak  solution  and  proceed  gradu- 
ally. The  proper  development  of  each  plate  will  take  at  least 
an  hour.  Undoubtedly  many  well  exposed  coronal  photo- 
graphs have  been  spoiled  through  improper  care  in  develop- 
ment. The  technique  of  development  of  photographs  is  so 
well  known  that  no  attempt  at  further  details  will  be  given 
here. 

In  reviewing  the  scientific  work  accomplished  at  solar 
eclipses,  one  is  immediately  struck  by  the  fact  that  success 
has  been  greater  in  the  direct  photography  of  the  corona  than 
in  any  other  branch  of  eclipse  investigations.  The  self-evident 
reason  is  that  every  well-equipped  eclipse  expedition,  no  mat- 
ter what  their  other  program,  attempts  to  photograph  the 
corona.  However,  there  is  another  reason  not  so  apparent, 
which  is  that  perfection  of  focus  and  seeing,  although  de- 
sirable, are  not  absolutely  essential  in  obtaining  successful 


364  ECLIPSES  OF  THE  SUN 

photographs  of  coronal  details.  A  lack  of  perfect  focus  plays 
havoc  with  spectroscopic  photographs  but  detracts  little  from 
the  corona  for  the  reason  that  its  structure  is  nebulous  or 
filmy  in  detail.  These  remarks  should  not  be  interpreted 
to  mean  that  one  should  be  satisfied  with  anything  short  of 
absolute  perfection  in  the  determination  of  the  best  focus. 

The  general  form  of  the  corona  can  be  predicted  in  advance 
of  the  eclipse.     At  sun-spot  minimum  are  found  the  long 
equatorial  streamers  and  the  short  plume-like  polar  brushes 
which  were  well  seen  at  the  eclipse  of  1900  (p.  164),  or  in 
1922  (p.  365).    At  sun-spot  maximum  the  corona  is  nearly 
circular  in  shape,  thus  resembling  a  gigantic  dahlia.     How 
closely  are  these  typical  shapes  connected  with  the  sun-spot 
curve?    It  is  rather  curious  to  find  that  in  the  thirty  years 
following  1893,  when  large-scale  photographs  of  the  corona 
were  first  successful,  sixteen  total  eclipses  were  observed 
either  near  maximum  or  minimum  of  spots,  or  were  distrib- 
uted along  the  branch  of  the  curve  descending  from  maxi- 
mum.   The  eclipse  of  1914  was  the  only  corona  seen  on  the 
ascending  branch  of  the  sun-spot  curve,  the  corona  of  1916 
having  been  obscured  by  clouds.    Fortunately  for  our  knowl- 
edge of  this  subject,  the  eclipses  of  1925,  1926  and  1927  were 
all  on  the  ascending  part  of  the  curve.    With  the  eclipses  of 
1929  and  1930  on  the  descending  branch  of  the  sun-spot  curve 
near  maximum,  our  information  regarding  the  shape  of  the 
corona  can  now  be  subjected  to  more  exact  scrutiny.    We  ask 
ourselves  the  question  whether  the  typical  coronal  shapes 
take  place  exactly  at  maximum  and  minimum,  or  whether 
they  start  before  or  after  the  times  of  maximum  and  mini- 
mum.   Take  one  particular  eclipse.    Sun-spot  maximum  oc- 
curred in  August,  1917.    The  eclipse  of  June,  1918,  did  not 
show  the  typical  corona  of  sun-spot  maximum,  for  the  polar 
streamers  were  shorter  than  those  near  the  equator,  and  the 
corona  departed  more  from  the  circular  shape  than  was  ac- 
tually anticipated.    Generally  when  the  maximum  of  spots 
is  past,  the  streamers  draw  away  from  the  poles,  and  the  long- 
est rays  are  found  in  the  sun-spot  zones,  making  the  corona 
rectangular  in  appearance.     . 

Although  it  may  be  said  with  truth  that  the  shapes  of  the 


Spain 


1918 
\merica 


1908 
Flint 
Island 
Pacific 


1922 
Australia 


THREE  CORONAS  PHOTOGRAPHED  BY  LICK  ASTRONOMERS  AND  ONE  (1922)  BY 
CANADIAN  OBSERVERS 


THE  CORONA  365 

corona  at  minimum  of  sun-spots  all  resemble  each  other  in 
having  long  equatorial  streamers  and  pronounced  polar 
brushes,  yet  each  corona  has  its  own  particular  features,  its 
own  peculiar  structure.  The  coronas  of  1878,  1889,  I9°° 
and  1922,  all  taken  at  minimum  phase  of  sun-spots,  could 
never  be  mistaken  one  for  the  other.  In  fact  there  were  two 
eclipses  in  the  year  1889,  on  January  i  and  December  22, 
with  very  pronounced  alterations  in  shape  in  the  coronas. 
These  variations  in  form,  as  pointed  out  by  Wesley,1  were 
precisely  in  accordance  with  the  change  to  be  expected  with 
the  sun  returning  toward  a  condition  of  spot  activity.  The 
shapes  of  the  coronas  from  1922  to  1927,  inclusive,  furnish 
interesting  comparisons.  The  eclipse  of  September  14,  1922 
represented  the  typical  minimum  corona.  The  eclipse  of 
September  10,  1923  exhibited  the  same  features.  Even  on 
January  24,  1925,  there  was  still  the  typical  minimum  shape, 
of  pronounced  polar  brushes,  but  the  equatorial  streamers 
were  not  so  long  as  those  of  1922.  To  the  right  of  the  verti- 
cal, however,  there  was  a  long  pointed  shaft  of  light  giving 
mute  evidence  that  the  time  of  minimum  was  well  past.  On 
January  14,  1926,  the  corona  had  lost  all  of  the  characteris- 
tics of  the  minimum  type  and  closely  resembled  that  associ- 
ated with  maximum  of  spots,  though  the  time  was  two  and 
one  half  years  before  the  expected  maximum  of  spots.  The 
corona  of  June  29,  1927  was  distinctly  that  of  maximum 
type. 

Corona  times  near  sun-spot  maxima  resemble  each  other 
even  less  than  do  the  different  aureoles  at  sun-spot  minima. 
The  coronas  corresponding  to  maximum  of  spots  are,  it  is 
true,  approximately  circular  in  outline,  but  prominent  rays 
and  streamers  shoot  out  at  various  angles.  We  are  forced 
therefore  to  the  conclusion  that  changes  are  continuously  go- 
ing on  in  the  corona.  How  rapid  are  these  changes?  Can  they 
be  detected  from  photographs  taken  with  the  same  camera, 
at  the  beginning  and  ending  of  a  total  eclipse?  It  is  possible 
that  the  few  short  minutes  of  totality  may  afford  too  short  an 
interval  to  permit  these  changes  to  be  detected  with  cer- 
tainty, from  photographs  taken  at  any  one  location,  but  mo- 

1  Observatory,  13,  105,  1890. 


366  ECLIPSES  OF  THE  SUN 

tions  might  possibly  be  detected  from  photographs  secured 
at  widely  separated  stations  at  the  same  eclipse. 

Any  theories  dealing  with  coronal  structures  must  depend 
on  the  measured  changes  within  the  corona  itself ,  and  it  is 
therefore  imperatively  necessary  that  reliable  information 
be  secured  regarding  these  motions.  The  importance  of 
this  problem  has  been  fully  recognized  by  eclipse  observers 
for  many  a  long  year.  Manifestly,  no  real  progress  was 
possible  as  long  as  it  was  necessary  to  depend  for  informa- 
tion on  drawings  of  the  corona.  In  fact,  sketches  made  dur- 
ing the  time  of  an  eclipse  have  been  a  continued  disappoint- 
ment, but  the  non-success  was  no  greater  than  what  should 
have  been  expected.  Even  a  skillful  draughtsman  subjected 
to  the  excitement  and  unfamiliarity  of  a  total  eclipse  could 
not  be  expected  to  see  and  draw,  in  the  hurried  interval  of 
a  couple  of  minutes,  more  than  a  few  details  of  coronal  struc- 
ture. These  details  of  necessity  must  be  exaggerated  in  the 
drawing.  A  sketch  made  by  a  second  person,  as  well  trained 
as  the  first  draughtsman  and  secured  at  the  same  time  and 
location,  probably  would  stress  other  details  regarded  as 
important.  But  to  make  matters  worse,  most  of  the  draw- 
ings of  the  corona  made  in  the  past  were  secured  by  men 
who  were  observing  their  first  eclipse,  and  frequently  these 
very  men  had  little  experience  or  skill  in  the  making  of 
sketches.  With  the  advent  of  the  photographic  plate,  the 
wholesale  drawing  of  the  corona  during  totality  has  been 
pushed  more  and  more  into  the  background. 

To  detect  motions  in  the  corona  with  the  greatest  cer- 
tainty, several  prerequisites  are  necessary:  First,  the  photo- 
graphs to  be  measured  should  be  on  as  large  a  scale  as  pos- 
sible; second,  the  interval  in  time  should  be  as  great  as 
possible;  and  third,  the  photographs  to  be  compared  should 
be  secured  with  cameras  of  nearly  the  same  focal  length  and 
they  should  resemble  each  other  in  general  appearance  as 
much  as  possible.  Plates  developed  with  different  effects  of 
contrast  and  taken  under  different  conditions  of  seeing  cannot 
furnish  motions  with  the  highest  degree  of  precision. 

Attempts  were  made  as  early  as  1889  to  secure  the  neces- 
sary information  by  the  comparison  of  photographs  at  the 


THE  CORONA  367 

eclipse  of  December  22.  An  interval  of  two  and  a  half  hours 
elapsed  between  the  time  of  totality  at  Cayenne  and  Cape 
Ledo  in  West  Africa.  Unfortunately,  clouds  were  experi- 
enced at  the  latter  station.  As  a  result  of  the  long  duration 
of  totality  at  the  eclipse  of  1901,  an  exceptionally  favorable 
opportunity  existed  for  determining  motions  from  observa- 
tions at  a  single  station.  Perrine1  secured  successful  photo- 
graphs with  the  40-foot  camera.  Measures  of  short  exposure 
negatives,  taken  near  the  beginning  and  end  of  totality, 
showed  no  displacements  of  coronal  masses  in  the  interval 
of  a  little  more  than  five  minutes.  On  account  of  the  accu- 
racy of  the  measurements,  it  was  possible  to  have  measured 
with  certainty  a  velocity  of  20  miles  per  second  across  the  line 
of  sight.  Motions  should  have  been  suspected  if  they  had 
been  as  great  as  12  or  15  miles  per  second.  The  1901  pho- 
tographs were  near  sun-spot  minimum.  At  the  eclipse  of 
1905,  Hansky  secured  photographs  on  the  same  scale  as  Per- 
rine's  photographs/  The  corona  was  one  of  typical  maxi- 
mum form  and  presented  few  conditions  for  the  best  results. 
The  corona  was  very  intense,  especially  near  the  solar  sur- 
face, and  its  rays,  being  projected  from  all  sides  of  the  disk 
of  the  sun,  so  superposed  themselves  the  one  upon  the  other 
and  became  so  entangled  that  it  was  almost  impossible  to 
distinguish  the  same  details  on  successive  photographs.  By 
taking  the  original  negatives  and  making  glass  positives,  by 
proper  shading  and  by  local  development,  much  detail  was 
secured  in  the  corona.  From  the  measures  of  43  separate 
rays,  information  was  obtained  concerning  the  motions  of 
the  coronal  material  within  the  period  of  totality  lasting  a 
little  more  than  three  minutes.  The  velocities  determined 
were  little  greater  than  the  errors  of  observation.  None  of 
the  velocities  investigated  exceeded  25  km  per  second. 

In  accordance  with  the  plans  of  the  Lick  Observatory  of 
always  attacking  eclipse  problems  of  the  greatest  importance, 
an  attempt  was  made  at  the  eclipse  of  1905  to  detect  changes 
in  the  coronal  structure  by  establishing  three  stations  at 
widely  separated  localities  and  securing  large-scale  photo- 
graphs at  each  station.  Parties  were  sent  to  Labrador,  to 

1  Lick  Observatory  Bulletin,  i,  151,  1902.  2  Mitt.  Pulk.,  2,  No.  19,  107. 


368  ECLIPSES  OF  THE  SUN 

Spain  and  to  Egypt.  Unfortunately  for  the  success  of  the 
scheme,  cloudy  weather  prevailed  in  Labrador  where  were 
stationed  Curtis  and  Stebbins.  Thin  clouds  were  met  in 
Spain,  but  they  did  not  greatly  interfere  with  the  photo- 
graphs secured  by  Campbell  and  Perrine.  Clear  skies  greeted 
Hussey  in  Egypt  where  totality  occurred  70  minutes  later 
than  in  Spain.  A  careful  comparison  was  made  of  the  photo- 
graphs1 secured  at  the  two  stations.  A  number  of  fairly 
well-defined  nuclei  were  found,  both  east  and  west  of  the 
sun.  Details  of  structure  within  the  nuclei  appeared  to 
change,  but  the  nuclei  as  a  whole  remained  in  the  same  posi- 
tion. Measures  of  the  greatest  accuracy  were  impossible 
on  account  of  the  poor  definition  of  the  Egyptian  plates 
caused  by  poor  seeing.  The  conclusion  was  that "  the  masses 
in  question  could  not  have  moved  so  much  as  one  mile  per 
second  during  the  interval  of  4200  seconds.  Greater  speeds 
might  well  have  occurred  within  the  principal  coronal  stream- 
ers, or  within  some  of  the  arched  forms  which  enclose  promi- 
nences, without  our  having  detected  them;  for  their  structure 
is  quite  uniform,  and  well-defined  nuclei  are  absent/7 

The  American  eclipse  of  1918  afforded  an  additional  op- 
portunity by  comparing  plates  taken  at  three  separate  sta- 
tions where  were  located  expeditions  from  Lick,  Lowell  and 
Sproul  Observatories.  At  Goldendale,  Washington,  the  focal 
length  was  40  feet,  at  Syracuse,  Kansas,  the  camera  was  38.7 
feet  in  length,  while  at  Brandon,  Colorado,  the  camera  was 
63  feet  long.  One  plate  secured  by  each  of  the  first  two 
cameras  and  two  plates  with  the  63-foot  instrument  were 
compared.  Each  of  the  three  cameras  pointed  directly  at  the 
sun.  There  were  no  definite  nuclei  or  other  distinctive  fea- 
tures present  in  the  streamers  that  were  sufficiently  well  de- 
fined to  be  used  as  points  of  measurement  by  Miller."  There 
were,  however,  three  arches  surrounding  three  prominences 
and  attention  was  confined  to  these.  The  first  arch  was 
around  the  "  Pyramid  "  or  "  Eagle  "  prominence  in  the  north- 
eastern quadrant.  Towards  the  pole  side  of  this  prominence 
there  were  four  well-defined  arches.  The  second  arch  was 
around  a  prominence  in  the  SE  quadrant,  while  the  third 

1  Lick  Observatory  Bulletin,  4,  121,  1907.  2  Publ.  A.  S.  P.,  32,  207,  1920. 


THE  CORONA  369 

arch  was  near  the  "  Heliosaurus  "  prominence.  Each  of 
the  three  prominences  displayed  four  separate  arches,  and 
the  positions  of  one  arch  only  of  each  were  measured.  These 
measures  gave  fairly  accordant  results  and  seemed  to  show 
that  the  arches  had  changed  in  the  twenty-six  minutes'  in- 
terval between  the  Lick  and  the  Sproul  photographs.  The 
average  rate  of  speed  at  which  these  arches  receded  from  the 
sun  was  about  ten  miles  per  second. 

At  the  eclipse  of  1926,  Horn-D'Arturo  compared  photo- 
graphs taken  by  the  Italian  expedition  in  East  Africa  with 
those  taken  by  the  British  expedition  in  Sumatra  two  and  a 
half  hours  later.  He  measured  the  coronal  domes  at  position 
angle  335°  and  found  that  if  they  moved  at  all,  their  motions 
were  not  greater  than  2  km  per  second.  Von  Kliiber l  reached 
a  similar  conclusion  from  measures  on  the  photographs  of  the 
German  expedition  in  Sumatra  compared  with  Horn-D'Ar- 
turo's  measures. 

Horn-D'Arturo  found  that  on  the  photographs  taken  in 
Africa  there  were  well-defined  streamers  at  position  angles 
350°  and  320°,  both  traceable  to  heights  greater  than  one 
solar  radius,  whereas  on  the  Sumatra  photographs  the 
streamer  at  350°  had  disappeared,  the  streamer  at  320°  was 
intensified  and  a  new,  well-defined,  very  straight  streamer 
had  appeared  at  325°,  traceable  to  a  height  of  ii  solar  radii. 
If  the  new  streamer  be  interpreted  as  matter  just  ejected 
from  the  sun,  then  the  matter  must  have  been  moving  at 
least  50  km  per  second. 

From  the  photographs  compared  by  Miller,  it  was  evident 
that  the  polar  rays  east  of  the  axis  of  symmetry  are  curved 
away  much  less  than  the  rays  to  the  west,  the  difference 
not  being  due  to  the  effect  of  projection.  It  is  therefore 
evident  that  the  velocities  with  which  material  is  ejected 
from  the  sun  to  form  the  coronal  streamers  must  be  very 
small  compared  with  the  motions  with  which  we  are  fa- 
miliar in  the  prominences.  Any  theory  dealing  with  the 
formation  of  the  coronal  structure  must  take  cognizance 
of  these  moderate  velocities  of  ejection. 

The  corona  owes  its  entrancing  beauty  to  the  far-flung, 

1  Zcits.  jur  Astrophysik,  4,  i,  1932. 


370  ECLIPSES  OF  THE  SUN 

pearly-white  streamers,  so  conspicous  in  each  corona  ob- 
served. The  contrast  with  the  rosy-red  prominences  close 
to  the  edge  of  the  black  moon  makes  a  never-to-be-forgotten 
spectacle.  In  addition  to  the  streamers,  there  are  other 
features  of  the  corona  that  deserve  attention,  and  it  is  neces- 
sary to  find  out  their  relations  to  solar  phenomena  such  as 
prominences,  sun-spots  and  faculae.  "  Arches,"  "  hoods  " 
or  "  striated  cones  "  have  been  observed  since  Cleveland 
Abbe  first  saw  them  at  the  eclipse  of  1869.  They  are 
specially  conspicuous  at  the  time  of  sun-spot  maximum. 
The  first  good  photographs  obtained  of  them  were  by 
Schaeberle  at  the  eclipse  of  April,  1893.  The  coronal  arches 
were  noted  by  him1  to  be  associated  with  prominences. 
Miss  Clerke"  draws  the  conclusion  that  "  Each  pearly  pa- 
vilion is  erected  over  a  red  flame.  Coincidences  of  the  kind 
are  of  perpetual  occurrence."  These  hoods  were  specially 
marked  in  the  coronas  of  1896  and  1898,  but  were  prac- 
tically missing  from  the  minimum  type  of  corona  of  the 
year  1900. 

At  the  eclipse  of  1901,  a  different  kind  of  "  disturbance  " 
was  noted  in  the  photographs  secured  by  Perrine,  which 
consisted  of  a  conspicuous  center,  apparently  at  the  sun's 
limb,  from  which  strong  streamers  stretched  out  to  great 
distances  from  the  edge  of  the  sun.  This  disturbance  was 
all  the  more  interesting  for  the  reason  that  Perrine  found 
it  to  take  its  origin  over  the  region  of  a  prominent  sun-spot. 
In  fact,  this  spot  was  the  only  one  known  to  exist  on  the  sun 
at  the  time.  Similar  disturbances  were  seen  in  the  photo- 
graphs of  the  eclipse  of  1918,  but  Campbell  and  Moore 
could  find  no  relation  between  them  and  any  sun-spot  or 
chromospheric  phenomena  known  to  exist  on  the  sun.  As 
already  stated,  the  arches  were  specially  prominent  at  this 
recent  eclipse  and  they  extended  to  much  greater  distances 
from  the  sun's  limb  than  they  did  in  1893,  although  both 
eclipses  were  near  the  sun-spot  maximum.  In  1923,  Miller 
photographed  a  disturbed  area  in  the  corona  intimately  asso- 
ciated with  a  spot  near  the  sun's  limb,  shown  at  page  209.  As 


1  Contributions  from  the  Lick  Observatory ,  4,  94,  1895. 

2  Problems  in  Astrophysics,  129,  1903. 


THE  CORONA  371 

a  whole,  disturbances  are  much  more  conspicuous  in  the  coro- 
nas seen  at  maximum  of  sun-spots  for  the  reason  that  promi- 
nences are  much  more  numerous  and  active  and  are  found  in 
all  heliographic  latitudes,  while  at  sun-spot  minimum  the 
prominences  are  feeble  and  are  confined  to  zones  near  the 
solar  equator. 

Much  valuable  information  regarding  coronal  disturbances 
was  obtained  3  at  Niuafoou  Island  in  1930.  This  was  the  first 
time  in  the  history  of  eclipses  that  so  much  detailed  structure 
was  visible  in  the  spectral  lines  of  the  corona,  this  structure 
being  made  possible  by  the  fine  definition  of  the  concave  grat- 
ing spectra  taken  without  slit.  A  comparison  of  all  the  1930 
grating  spectrograms,  but  more  particularly  a  study  of  the 
H  and  K  lines,  reveals  the  interesting  fact  that  the  sun,  two 
years  after  sun-spot  maximum,  was  in  a  condition  of  great 
activity  on  the  day  of  the  eclipse.  The  spectra  and  the 
direct  photographs  taken  by  the  63 -foot  tower  telescope  by 
Marriott  show  prominences  completely  encircling  the  sun. 
Comparisons  were  also  made  with  spectroheliograms  on  four 
successive  days,  taken  at  Mt.  Wilson  or  Kodaikanal.  The 
superb  definition  of  the  eclipse  photographs  shows  both  the 
prominences  and  the  inner  corona  in  great  beauty. 

A  comparison  of  all  the  photographs,  the  spectrohelio- 
grams with  the  eclipse  plates  both  direct  and  spectrographic, 
shows  that  the  sun  was  very  active  not  only  at  eclipse  time 
but  throughout  the  whole  period  of  nearly  four  days  covered 
by  the  plates.  At  the  time  of  totality  there  was  a  promi- 
nence almost  exactly  at  the  south  point  of  the  sun  extending 
to  a  height  of  25,000  km.  At  this  time  the  axis  of  rotation 
passed  26°  to  the  west  of  the  south  point.  The  photographs, 
direct  and  spectrographic,  show  prominences  even  at  the 
north  and  south  poles  of  the  sun's  axis. 

Marriott's  photographs  and  the  eclipse  spectra  exhibit 
that  the  whole  southeast  quadrant  was  a  tremendously 
stormy  region  on  the  sun.  The  center  of  the  longest  stream- 
ers of  the  whole  corona  was  situated  30°  to  the  east  of  the 
south  point  of  the  sun,  or  at  position  angle  150°.  Inter- 
twining the  coronal  streamers  is  a  beautiful  series  of  coronal 

1  Publ.  Leander  McCormick  Obsy.,  5,  155;  Astroph.  Jour,,  75,  i,  1932. 


;j72  ECLIPSES  OF  THE  SUN 

domes.  At  totality,  at  the  base  of  the  long  streamers  and 
domes  is  an  extended  group  of  prominences  centered  at  150° 
of  position  angle.  These  prominences,  though  apparently 
very  active  as  shown  by  the  eclipse  photographs,  did  not 
reach  the  greatest  heights  on  the  sun.  These  greatest  heights 
were  actually  attained  by  prominences  also  in  the  southeast 
quadrant  but  at  115°  of  position  angle.  This  is  the  location 
of  the  conspicious  feature  called  the  "  strawberry  dome,"  with 
its  magnificent  series  of  arches  and  with  its  delicate  inter- 
twined structure  of  filamentous  details.  Facing  page  373,  at 
A  and  B  are  found  drawings  from  the  eastern  edge  of  the 
sun  from  the  region  where  the  "  strawberry-shaped  "  dis- 
turbance is  found  on  Marriott's  photographs.  At  A  are 
drawings  from  the  K  and  Ha  lines  of  the  spectra  of  the 
chromosphere,  and  at  B  is  the  same  region  from  the  direct 
photographs  and  from  the  lines  5303  and  6374  of  coronium. 
At  C  on  the  western  edge  of  the  sun,  the  detail  in  the  5303 
line  resembled  the  cocoanut  trees  which  grew  in  great  pro- 
fusion on  "  Tin-Can  "  Island. 

For  many  long  years  all  eclipse  observers  have  called  at- 
tention to  the  connection  between  coronal  streamers  and 
prominences.  This  dependence  of  streamers  and  prominence 
activity  is  abundantly  verified  in  1930,  but  this  eclipse  dem- 
onstrates the  fact  that  the  longest  coronal  streamers,  on 
which  the  shape  of  the  corona  more  or  less  depends  are  not 
necessarily  connected  with  the  prominences  which  at  the 
time  of  the  eclipse  are  of  greatest  height. 

The  long  streamers  of  the  corona  are  best  seen  near  the 
time  of  minimum  of  sun-spots.  The  splendid  series  of  photo- 
graphs secured  by  Lick  Observatory  expeditions  have  been 
utilized  by  Moore  and  Baker  to  measure  the  direction  of  the 
axis  of  symmetry  of  the  polar  brushes,  photographs  at  five 
separate  eclipses  exhibiting  the  polar  rays  sufficiently  well- 
determined  for  the  purpose.  The  direction  of  the  sun's 
equator  is  inclined  about  7°  to  the  ecliptic  and  26°  16'  to 
the  terrestrial  equator.  The  measures  given  in  the  following 
table  show  that  the  axis  of  symmetry  of  the  polar  rays  coin- 
cides, within  errors  of  measurement,  with  the  rotation  axis 
of  the  sun. 


PQ 


I 


° 


s 


C        lH 

O    O 


W      M 


•a 


East 


DRAWINGS  MADE  BY  EMMA  T.  R.  WILLIAMS  FROM  SPECTRA  BY 
CONCAVE  GRATING  WITHOUT  SLIT 

A  and  B  refer  to  the  eastern  section  of  the  sun  shown  in  Plates  I  and  II, 
and  C  to  the  western  section.  (Please  note  that  all  positions  in  C  are  inverted.) 

In  A  are  given  the  K  line  of  Ca-f-  and  the  Ha  line  of  the  chromosphere 
from  spectra  taken  immediately  following  the  first  flash.  In  B  and  C  are 
given  (from  left  to  right)  details  from  Marriott's  photographs  near  the 
beginning  (B)  and  end  (C)  of  totality,  and  the  structure  in  the  coronal 
rings  of  spectra  at  X  5303  and  X  6374,  respectively. 


East 


West 


THE  CORONA 


373 


Axis  OF  SYMMETRY  OF  POLAR  STREAMERS  IN  SOLAR  CORONA 
(Position  angles  measured  from  North  point  of  Sun) 


Date  of  Eclipse 

Axis  of  Polar  Streamers 

Rotation 

Polar 
Streamers 
minus 

Moore 

Baker 

Mean 

Axis 

Rotation 
Axis 

o 

0 

0 

1898  Jan.    21 

9.4  W 

9-3  W 

9.4  W 

8.2  W 

1.2  W 

IQOO  May   28 

17.1  W 

17.6  W 

17.4  W 

16.9  W 

0.5  W 

1901  May    18 

21.  5  W 

20.7  W 

21.  1  W 

20.3  W 

0.8  W 

1908  Jan.      3 

0.9  W 

0.8  E 

0.0 

1.2   E 

1.2  W 

1922  Sept.  21 

24.4  E 

24.8  E 

24.6  E 

25.0  E 

0.4  W 

Average 

0.8  W 

Facing  page  373,  line  i  following  caption,  for  in  Plates  I 
mcl  IF  read  facing  pages  8  and  13 


granted  that  every  spectral  line  seen  at  mid-totality,  when 
to  the  eye  there  was  no  visible  trace  of  the  chromosphere, 
must  perforce  take  its  origin  in  the  corona.  It  was  with  a 
great  shock  of  surprise  that  H  and  K  of  calcium  were  seen  in 
1882  projected  on  the  black  face  of  the  moon,  where  presum- 
ably there  is  no  light  at  all.  About  thirty  bright  lines  were 
photographed  by  Schuster  and  by  Abney  at  the  eclipses  of 
1882,  1883  and  1886.  Even  as  late  as  1893,  the  H  and  K 
lines  were  assumed  to  belong  to  the  corona. 

Knowledge  of  the  spectrum  of  the  corona  may  almost  be 
said  to  begin  with  the  eclipse  of  1893,  when  Fowler  with  a 
prismatic  camera  photographed  nine  rings,  all  of  which  agree 
in  position  with  lines  reported  for  the  1886  eclipse.  At  the 
same  eclipse,  Deslandres  photographed  three  coronal  lines 
at  3987,  4086  and  4231  A,  and  also  three  others  at  3164, 
3170  and  3237  A,  in  the  ultra-violet.  These  last  three  have 
not  been  observed  since  by  others,  nor  have  two  of  Fowler's 
nine.  It  was  not  until  1898  that  coronal  spectra  had  acquired 
sufficient  precision  to  distinguish  between  the  chromospheric 


374  ECLIPSES  OF  THE  SUN 

line  1474  K  at  wave-length  5317  and  the  "  coronium  "  line 
fourteen  angstroms  farther  to  the  violet,  at  5303.  Photo- 
graphs were  secured  at  the  eclipse  by  Fowler,  by  Campbell, 
by  Naegamwala,  and  by  Newall  and  Hills.  Additional  pho- 
tographs were  secured  by  Frost  in  1900,  by  Dyson  in  1900, 
1901  and  1905,  by  the  U.  S.  Naval  Observatory  parties  in 
1900, 1901  and  1905,  by  Fowler  and  Lockyer  in  1900,  and  by 
Lockyer  and  by  Campbell  in  1905.  More  recently  the  Lick 
expeditions  have  secured  good  photographs,  by  Lewis,  and 
by  Campbell  and  Albrecht  in  1908,  and  by  Lewis,  by  Camp- 
bell and  Moore  in  1918  and  again  in  1922.  More  recently 
excellent  spectra  have  been  secured,  in  1925  by  Curtis  and 
Burns,  in  1926  by  Davidson  and  Stratton,  in  1929  by  Gro- 
trian,  and  in  1930  by  Mitchell. 

At  the  eclipse  of  1918,  Slipher  found  on  his  photographs 
the  green  coronium  line  extending  across  the  space  occupied 
by  the  moon's  image  —  due  to  scattering  of  light  in  the 
earth's  atmosphere.  At  the  eclipse  of  August,  1914,  Car- 
rasco,  and  Bosler  and  Block,  independently  discovered  a  new 
coronal  line  in  the  red,  at  wave-length  6374  A.  In  Lick  Ob- 
servatory Bulletin,  10,  i,  1918,  Campbell  and  Mopre  assem- 
ble all  the  reliable  observations  of  coronal  wave-lengths  de- 
termined since  the  eclipse  of  1893.  These  values  for  forty 
lines  are  given  in  the  second  edition  of  this  book  on  page  329. 
As  an  illustration  of  the  precision  of  knowledge  at  that  time 
regarding  coronal  wave-lengths  may  be  cited  the  measures 
for  the  strongest  line,  that  in  the  green.  Before  the  recent 
eclipse,  the  most  accurate  results  came  from  the  1918  obser- 
vations, by  Campbell  and  Moore,  and  by  Adams,  St.  John  and 
Miss  Ware.  Uncorrected  for  rotation  (0.035  A),  and  cor- 
rected (assuming  the  corona  rotates  at  the  same  rate  as  the 
photosphere ) ,  the  values  in  international  angstroms  are  given : 

Uncorrected        Corrected 

Campbell  and  Moore  5302.83  5302.80 

Adams,  St.  John  and  Ware   5303.02  5303.06 

The  difference  in  the  corrected  values  is  0.26  A,  while 
the  mean  of  the  two  measures  is  5302.93.  The  mean  value  is 
in  excellent  agreement  with  the  1930  result  of  5302.91  A 
from  the  measures  by  Mitchell  of  eight  spectra.  Six  spectra 


THE  CORONA  375 

at  the  1930  eclipse  give  6374.28  A,  for  the  line  discovered 
at  the  1914  eclipse. 

It  might  be  said  that  the  corona  exhibits  three  separate 
spectra:  first,  the  bright-line  spectrum  of  "  coronium  "  exist- 
ing only  in  the  inner  corona  and  extending  on  the  average  to 
about  5'  or  200,000  km  from  the  sun's  edge;  second,  the  con- 
tinuous spectrum  of  the  middle  corona;  and,  third,  the  Fraun- 
hofer  lines  showing  feebly  in  the  outer  corona. 

In  the  Revised  Rowland  Tables,  page  226,  1928,  is  given 
the  list  of  bright  lines  attributed  to  the  corona  by  Campbell 
and  Moore  but  revised  to  include1  the  1926  eclipse.  In 
Zeitschrijt  fur  Astrophysik,  2,  106,  1931,  Grotrian  gives  the 
results  of  the  Potsdam  expedition  to  Sumatra  of  May  9,  1929. 
In  the  following  table,  the  1930  eclipse  is  also  included. 
After  eliminating  the  lines  due  to  the  high  chromosphere, 
there  are  only  eighteen  lines  out  of  forty  which  are  now 
believed  to  be  truly  coronal  in  origin,  although  a  line  or  two 
may  still  be  suspicious.  The  first  column  gives  the  wave- 
lengths in  international  units.  Those  at  the  top  of  the  table, 
given  to  two  places  of  decimals,  are  from  the  1926  eclipse 
by  Davidson  and  Stratton.  The  values  of  5302.91  and 
6374.28  are  from  the  1930  spectra.  The  line  at  6704  was 
discovered  at  the  1929  eclipse  and  verified  by  the  1926  eclipse 
photographs.  The  line  at  6776  announced  by  Mitchell  at  the 
1930  eclipse  needs  to  be  verified.  The  intensities  given  in 
the  remaining  columns  are  estimates  where  the  strongest 
lines,  3388  in  the  violet  and  5303  in  the  green,  are  usually 
assumed  to  be  of  intensity  20.  The  intensities  given  in  the 
table  are  those  assigned  by  the  observers  except  for  the  1918 
eclipse  where  Moore  and  Campbell's  estimates  are  each  mul- 
tiplied by  the  factor  2  so  that  the  line  5303  may  have  an 
intensity  20.  For  the  1922  eclipse,  the  intensities  have  been 
furnished  by  letter  through  the  kindness  of  Moore.  In  the 
second  column  an  average  intensity  is  assigned  from  the  mean 
of  all  the  eclipses. 

The  intensities  of  the  coronal  lines  in  the  table  must  be 
considered  to  be  on  a  relative  scale  and  not  absolute  for  the 

1  Davidson  and  Stratton,  Memoirs  of  the  Royal  Astronomical  Society,  64,  105, 
1927. 


376 


ECLIPSES  OF  THE  SUN 


reason  that  5303  varies  from  eclipse  to  eclipse  and  is  not  con- 
stant as  was  assumed  in  forming  the  tabular  values. 

Before  taking  up  the  striking  differences  in  the  intensities 
from  eclipse  to  eclipse  shown  by  the  table,  one  must  consider 
the  factors  upon  which  the  coronal  intensities  depend.  They 
may  be  grouped  under  four  heads:  (i)  color  sensitivity  of 
the  photographic  plate,  (2)  the  dispersion,  (3)  prism  or  grat- 
ing, and  (4)  slit  or  slitless.  The  improvements  in  plates 
in  recent  years,  making  them  more  sensitive  at  the  red  end, 
are  mainly  responsible  for  the  discovery  of  the  lines  in  the 
red.  Prisms  of  glass  absorb  at  the  violet  end  of  the  spectrum ; 

WAVE-LENGTHS  AND  INTENSITIES  OF  CORONIUM 


Wave- 
Length 
I  A. 

Mean 
In- 
ten- 
sity 

Hills 
and 
New- 
all, 
i8g6 

Fowler, 
Shackle- 
ton,  and 
Lockyer, 

1898 

Dyson 
Mean 
1900- 
1901  - 
1905 

Lewis, 
1908 

Moore 
and 
("amp- 
bell, 
1918 

Moore, 
1922 

David- 
son 
and 
Strat- 
ton, 
1926 

(iro- 
trian, 
1929 

Mitchell 

1905 

IQ2S 

1930 

«28           . 

8 

8 

^87  96.. 

20 

12 

30 

20 

20 

20 

20 

20 

3454  U 

8 

9 

8 

5 

8 

1C 

5 

6 

3600.97.. 

10 

IS 

10 

6 

9 

12 

is 

IS 

^642  87.. 

3 

2 

5 

4 

3 

\ 

t 

4 

3800  77 

3 

3 

3 

} 

3 

2 

4 

I 

3 

$ 

} 

3986  88  . 

8 

5 

5 

3 

4 

2 

4 

10 

S 

8 

4 

8 

4086  2Q    . 

6 

6 

2 

2 

8 

6 

4 

6 

42*1.4.     • 

8 

10 

5 

5 

8 

6 

s 

10 

8 

6 

6 

6 

43  1  1 

2 

2 

i 

2 

i 

4550 

4 

3 

3 

4 

4 

i 

2 

2 

4567 

4 

8 

3 

6 

2 

6 

3 

4586 

2 

I 

4 

4 

5H7 

2 

2 

i 

2 

S 

^02  91    . 

20 

8 

1C 

20 

10 

20 

20 

20 

20 

20 

20 

20 

5536 

I 

2 

2 

6m  28 

12 

5 

6 

IS 

IS 

6704 

2 

4 

gratings  (of  speculum  metal)  and  the  silver-on-glass  re- 
flectors absorb  in  the  ultra-violet.  Increase  of  dispersion 
spreads  the  monochromatic  images  farther  apart  and  weakens 
the  continuous  spectrum,  with  the  result  that  the  emission 
lines  by  contrast  are  easier  to  see  on  the  photographs  taken 
with  higher  dispersion. 

At  the  1930  eclipse  on  Niuafoou  there  was  a  striking  dif- 
ference between  the  coronal  spectra  taken  with  the  concave 
gratings  and  with  the  prismatic  camera  of  Dr.  C.  E.  Adams 
of  the  New  Zealand  expedition.  The  prismatic  camera  gave 
a  very  brilliant  spectrum  —  in  fact,  more  brilliant  than  that 
of  either  concave  grating.  Both  forms  of  spectrographs  were 


THE  CORONA  377 

without  slits.  The  dispersion  of  the  gratings,  however,  was 
ten  times  the  average  dispersion  of  the  prismatic  spectrum. 
Many  coronal  rings  were  seen  on  the  grating  photographs, 
the  lines  5303  and  6374  being  well  visible  even  on  the  brief 
exposure  for  the  second  flash.  Oft  the  prismatic  spectra,  on 
the  contrary,  the  only  coronal  line  visible  on  any  of  the  photo- 
graphs was  5303,  and  even  this  line  was  seen  with  great 
difficulty.  It  is  readily  seen  therefore  that  the  dispersion 
employed  has  a  very  important  bearing  on  the  intensities,  and 
hence  any  conclusions  drawn  from  variations  in  intensities 
between  the  lines  of  one  eclipse  and  another  have  very  little 
meaning  unless  the  instrumental  conditions  are  fairly  con- 
stant. 

The  specially  interesting  features  of  the  coronal  spectra 
of  1930  are  the  large  dispersion  and  absence  of  slit.  On  the 
photographs,  the  image  of  the  sun  is  14.5  mm  in  diameter  and 
the  dispersion  10.9  angstroms  per  mm.  All  of  the  coronal 
rings  show  much  interesting  detailed  structure,  but  this  is 
specially  true  for  the  strong  5303  and  6374  rings.  On  the  cuts, 
facing  page  372,  the  vertical  line  coincides  approximately 
with  the  axis  of  rotation  of  the  sun.  One  can  see  at  a  glance 
that  if  a  slit  had  been  used  in  1930  passing  radially  through 
the  center  of  the  sun,  the  intensity  of  5303  (or  6374)  would 
have  depended  on  the  position  angle  of  the  slit.  If  placed 
near  the  solar  axis,  the  coronal  lines  would  have  been  feeble 
while  if  it  had  happened  to  pass  over  a  more  brilliant  portion 
(which  could  not  have  been  told  in  advance)  the  intensity 
would  have  been  much  greater.  Hence,  for  discussing  the 
distribution  of  coronium  about  the  sun,  slitless  spectra  (if 
obtained  with  large  dispersion  and  good  definition)  are  vastly 
superior  to  spectra  taken  with  a  slit. 

With  these  instrumental  sources  of  variations  in  intensities 
clearly  in  mind,  let  us  look  at  the  estimates  of  intensities  in 
the  foregoing  table.  There  are  several  striking  peculiarities. 
Many  of  these,  and  others,  have  already  been  noted  by  Camp- 
bell and  Moore.  The  most  remarkable  involves  the  line  at 
3601  A.  It  was  not  observed  by  any  one  until  the  eclipse 
of  1908,  in  spite  of  the  fact  that  it  is  one  of  the  strongest  lines 
of  the  coronal  spectrum  and  the  quartz  spectrographs  were 


378  ECLIPSES  OF  THE  SUN 

adequate  to  secure  it.  The  photographs  by  Mitchell  at  the 
three  eclipses  of  1905,  1925  and  1930  were  all  taken  with  the 
same  spectrograph.  The  plates  of  the  three  eclipses  have 
about  the  same  sensitivity  in  the  violet.  No  trace  of  3601  is 
found  in  1905  although  3388  and  3454  farther  to  the  violet 
are  strong  lines.  By  way  of  contrast,  in  1925  and  1930  the 
line  3601  was  very  strong.  (Other  peculiarities  of  this  line 
will  be  referred  to  later.)  Likewise,  4086  was  not  observed 
before  1908.  At  the  1929  eclipse  Grotrian  reports  three  lines 
missing,  3643,  4586  and  5536.  The  last  line  of  the  three 
is  the  weakest  of  the  whole  coronium  spectrum;  it  is,  more- 
over, near  a  Fe  +  line  in  the  chromosphere  of  5534.85  and 
intensity  15  in  the  flash  spectrum.  Davidson  and  Stratton 
ascribe  this  line  to  the  corona.  The  first  of  the  three  lines  at 
3643  is  on  the  average  a  fairly  strong  line.  Mitchell  could 
not  be  sure  of  it  from  his  1925  spectra,  however,  though  it 
was  well  visible  in  1905  and  1930. 

One's  first  thought  presented  for  an  explanation  of  these 
peculiarities  is  that  the  intensities  vary,  and,  having  settled 
this  to  one's  satisfaction,  the  next  thing  is  to  try  and  con- 
nect the  variation  with  the  sun-spot  cycle.  There  we  strike 
a  snag.  The  years  1900  and  1901  had  coronas  of  minimum 
type  while  the  1905  eclipse  was  of  maximum  type,  and  yet 
3601  was  invisible  in  all  three  years  and,  moreover,  it  has 
been  a  strong  line  in  every  eclipse  without  exception  since 
1908, 

The  first  step  towards  finding  the  origin  of  the  bright 
lines  in  the  corona  evidently  will  be  to  explain  the  peculiari- 
ties of  intensities  just  noted  by  finding  relationships  between 
the  different  lines.  To  this  end,  Stratton  and  Davidson  ' 
summarize  the  similarities  of  coronal  structure  noted  pre- 
viously by  others.  Lockyer  and  Fowler  place  4086,  4231, 
4400,  4586  and  5303  in  one  group;  3801,  3987  and  4567  in 
a  second;  and  4359  in  a  third.  Campbell  and  Moore  have 
three  groups:  3388,  3601  and  5303;  3801,  3987  and  4567; 
and  4231  by  itself  in  the  third  group.  Davidson  and  Stratton 
grouped  3388  with  3987;  3454  with  3643;  and  3601  with 
4086  and  5303.  They  further  suggest  that  "  3643  should  go 

1  Observatory,  54,  197,  1931. 


THE  CORONA  379 

with  3801  and  3987,"  thus  making  one  group  of  3388,  3454? 
3643,  3801  and  3987. 

The  1930  slitless  spectra  make  it  very  plain  that  coronium 
has  a  very  uneven  distribution  at  different  position  angles 
surrounding  the  sun.  The  limitations  of  the  spectra  taken 
with  a  slit  are  at  once  apparent.  If  the  slit  happens  to  fall 
on  an  intense  part  of  the  coronal  ring,  then  all  lines  belonging 
to  a  group  physically  connected  should  be  intensified  to- 
gether; but  if  the  slit  crosses  a  fainter  stretch  of  the  corona, 
all  lines  of  the  group  should  be  weakened  together.  Evi- 
dently the  comparisons  of  intensities  of  lines  at  one  eclipse 
with  those  at  another  (e.g.,  1926  and  1929)  may  mean  com- 
paratively little,  so  much  depends  on  the  position  angle  of 
the  slit.  Unfortunately,  for  the  purpose  of  discussing  groups, 
most  of  the  coronal  spectra  up  to  date  have  been  taken  with 
a  slit,  and,  moreover,  it  must  not  be  overlooked  that  the  slit- 
less  spectra  obtained  at  early  eclipses  have  left  much  to  be 
desired  in  the  matter  of  definition. 

As  already  noted,  the  line  3601  is  remarkably  peculiar, 
missing  in  1905  and  very  strong  in  both  1925  and  1930.  In 
the  last  two  years,  the  lines  are  very  intense  on  both  limbs 
of  the  sun  but  in  a  very  restricted  region  near  the  sun's  equa- 
tor. The  line  4086  resembles  3601  very  closely,  and  un- 
doubtedly these  form  a  pair.  With  either  one  or  the  other  of 
these  lines  the  following  have  been  grouped:  3388,  4231, 
4586  and  5303.  No  confirmation  of  this  grouping  is  given 
by  the  grating  spectra. 

Chief  interest  naturally  centers  in  the  green  line  at  530^. 
The  1930  spectra,  showing  details  that  in  shapes  resemble 
eruptive  prominences,  exhibit  the  identical  details  in  the  line 
3388  but  in  much  weaker  intensity  on  account  of  absorption 
by  the  speculum  of  the  grating  and  the  silver  of  the  coelostat. 
The  1905  coronal  spectra  likewise  show  structural  details 
similar  in  character  in  the  two  lines.  It  is  interesting  to  find 
the  two  strongest  lines,  3388  and  5303,  unquestionably  form- 
ing a  pair.  As  already  stated,  3601  and  4086  cannot  be 
grouped  with  this  pair. 

The  lines  3454  and  3643  certainly  do  not  form  a  pair,  the 
latter  being  missing  on  Grotrian's  1929  spectra  while  the 


380  ECLIPSES  OF  THE  SUN 

former  line  is  strong.  Mitchell's  1925  spectra  likewise  show 
3643  missing  or  doubtful.  There  seems  little  reason  for 
grouping  3643  with  3801  and  3987,  as  suggested  by  Stratton 
and  Davidson.  If  the  three  lines  form  a  group,  their  relative 
intensities  must  increase  and  decrease  together  in  different 
eclipses.  The  grating  spectra  and  also  those  of  Dyson  and 
of  Lewis  do  not  confirm  this  grouping. 

All  previous  authorities  have  made  a  group  of  3801,  3987 
and  4567.  The  relative  intensities  of  the  three  with  prismatic 
spectra  have  been  observed  as  follows:  (1896)  3,  5,  8; 
(1898)  3,  5,  3;  (1900,  1901,  1905,  Dyson)  3,  5,  6;  (1908) 
37  4,  invisible;  (1926)  i,  5,  3.  The  relative  intensities  vary 
so  enormously  from  eclipse  to  eclipse  that  it  does  not  look  rea- 
sonable to  combine  all  these  or  to  place  any  combination  of 
these  three  lines  into  pairs.  The  conclusion  is  confirmed  by 
the  slitless  grating  spectra.  After  examining  all  lines  of  the 
corona  there  seem  to  be  only  two  pairs  of  lines  which  appear 
to  show  similarities  in  form,  namely,  3388  and  5303,  and 
3601  and  4087. 

All  observers  who  have  discussed  slitless  coronal  spectra 
have  come  to  the  conclusion  that  the  green  ring  at  5303  is 
weakest  near  the  sun's  axis  and  strongest  in  the  sun-spot 
zones.  A  summary  of  the  information1  including  the  1930 
eclipse  shows  that  the  maximum  intensity  is  always  found 
near  prominences  but  does  not  necessarily  coincide  with  the 
prominences. 

In  comparing  the  images  of  5303  and  6374  in  the  1930 
spectra,  it  was  surprising  to  find  how  little  they  resembled 
each  other  in  their  structural  details  of  greatest  strength  and 
how  little  either  resembled  the  high-level  lines  of  K  and  Ha 
of  the  chromosphere.  Some  similarities  can  be  detected 
between  the  faint  streamers  in  the  lines  5303  and  6374  and 
those  in  Marriott's  direct  photographs,  but  in  the  stronger 
details  few  likenesses  can  be  found.  It  was  further  found 
that  the  radiation  of  6374  always  sticks  close  to  the  sun's 
edge  and  is  more  concentrated  and  more  uniform  than  5303. 
The  details  referred  to  above  are  found  in  the  drawings  re- 
produced at  pages  372  and  373.  From  these  comparisons  it 

1  Astro  physical  Journal,  75,  21,  1932. 


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THE  CORONA  381 

is  evident  that  5303  and  6374  cannot  take  their  origin  in  the 
same  atom,  or  at  least  not  in  the  same  atom  in  the  same  state 
of  ionization. 

On  account  of  the  apparent  simplicity  of  the  spectrum 
of  coronium,  Nicholson,  in  1911,  began  a  number  of  investi- 
gations of  great  interest,  the  results  of  which  have  appeared 
in  the  Monthly  Notices  of  the  Royal  Astronomical  Society. 
An  attempt  was  made  by  him  to  connect  the  wave-lengths 
by  series  relationships,  and  furthermore  to  find  an  explana- 
tion for  these  series  on  the  hypothesis  that  the  spectral  lines 
took  their  origin  in  an  atom  consisting  of  a  heavy  nucleus 
surrounded  by  negatively  charged  electrons.  Nicholson's 
work  was  the  first  attempt  to  explain  spectral  series  by 
means  of  Planck's  quantum  theory  of  radiation,  according 
to  which  interchange  of  energy  between  systems  of  a  periodic 
kind  can  take  place  only  in  certain  definite  amounts,  or 
quanta,  determined  by  the  frequencies  of  the  systems.  With 
the  enormous  increase  in  knowledge  of  atomic  structure  in 
the  past  twenty  years,  Nicholson's  investigations  have  for 
us  now  merely  a  historical  or  academic  interest. 

Pannekoek  1  has  assumed  that  the  coronal  lines  may  be 
due  to  Ca  +  -f  .  At  present  we  know  so  little  of  the  spectral 
lines  which  take  their  origin  in  the  singly-enhanced,  doubly- 
or  triply-enhanced  elements,  that  it  is  quite  futile  to  guess 
which  of  the  many  possible  elements  will  give  the  particular 
series  of  lines  seen  in  the  visible  portion  of  the  coronal  spec- 
trum. One  guess  is  just  about  as  good  as  another,  and 
therefore  we  shall  refrain  from  hazarding  a  conjecture.  The 
only  possible  method  of  securing  real  progress  is  to  improve 
the  observational  data  as  much  as  possible. 

Freeman "  believes  that  argon  is  the  cause  of  the  coronal 
lines.  He  finds  coincidences  in  wave-lengths  for  two-thirds 
of  the  forty  lines  in  Campbell  and  Moore's  list  (1918). 
Many  of  the  lines  identified  by  him  are  no  longer  regarded 
as  coronal  in  origin.  Russell  and  Bowen 3  come  to  the  con- 
clusion "  that  the  attribution  of  the  coronal  lines  to  argon 
is  without  foundation."  This  is  unquestionably  another 

1  B.  A.  N.,  14,  1922.  :{  A^tro physical  Journal,  68,  177,  1928. 

'2  Astrophysical  Journal,  69,  179,  1929. 


382  ECLIPSES    OF    THE    SUN 

case,  so  frequent  in  the  history  of  astrophysics,  of  the  identifi- 
cation of  origin  from  mere  coincidences  in  wave-length. 

Hylleraas L  criticizes  the  suggestion  by  Rosenthal  that  the 
coronal  spectrum  is  due  to  helium,  and  finds  there  is  no 
quantitative  agreement  to  support  this  theory. 

Hopfield "  finds  in  the  laboratory  spectrum  of  neutral  oxy- 
gen a  line  with  wave-length  6374.29  and  also  two  lines  at 
6300  and  6364  which  agree  in  wave-length,  within  limits  of 
error,  with  unidentified  lines  in  nebulae.  Several  workers 
have  produced  the  green  auroral  line  at  5577  in  the  labora- 
tory. There  is  a  remarkable  agreement  in  wave-length  be- 
tween the  laboratory  line  and  the  coronal  line  at  6374.28. 

More  recently,  de  Bruin  3  calculated  three  additional  lines 
in  the  neutral  oxygen  spectrum  with  wave-lengths  5302.70, 
6704.07  and  6775.90.  He  assumes  that  these  coincide  in 
wave-length  with  coronal  lines  and  hence  concludes  that "  the 
mysterious  coronium  turns  out  to  be  neutral  oxygen/7  For 
the  5303  line,  the  best  value  for  the  coronal  wave-length 
comes  from  taking  the  mean  of  the  1918  and  1930  eclipse  re- 
sults, which  gives  5302.92;  a  difference  too  large  to  permit 
identifying  the  oxygen  wave-length  with  the  coronal.  Still 
more  recently,  Frerichs  and  Dingle  in  Nature  (129,  901, 
1932)  independently  discuss  de  Bruin's  work.  The  former 
measures  the  wave-length  6374.292  for  the  oxygen  line. 
Both  of  these  physicists  come  to  the  conclusion  that  de 
Bruin's  coincidences  are  merely  accidental  and  that  "  the 
great  mystery  of  the  coronal  lines  remains  unsolved." 

In  addition  to  the  bright  line  spectrum,  the  corona  shows 
a  continuous  spectrum  in  the  inner  corona,  8'  or  10'  deep, 
with  Fraunhofer  absorption  lines  visible  in  the  middle  and 
outer  corona.  The  early  observations  seemed  to  indicate 
that  at  sun-spot  minimum  the  green  coronium  line  was  weak 
and  the  Fraunhofer  spectrum  strong,  while  at  sun-spot  maxi- 
mum the  emission  lines  were  stronger,  and  the  dark  lines 
weaker.  It  is  only  recently  that  observers  have  recognized 
that  the  presence  of  thin  clouds  at  the  time  of  the  eclipse 
may  greatly  affect  the  visibility  of  the  Fraunhofer  lines  ob- 

1  Zeit.  fur  Physik,  69,  361,  1931.    '  3  Nature,  129,  468,  1932. 

-  Physical  Review,  37,  160,  1931. 


THE  CORONA  383 

served  in  the  corona.  At  the  eclipse  of  1901,  photographed 
by  Perrine  in  Sumatra  through  thin  clouds,  the  coronal  spec- 
trum showed  Fraunhofer  lines  in  the  region  corresponding 
to  the  invisible  moon!  It  is  important,  for  testing  various 
theories  of  the  corona,  that  the  distance  out  from  the  sun  at 
which  the  absorption  lines  become  invisible  in  the  corona 
should  be  determined  by  adequate  photographs  taken  under 
clear  skies. 

A  great  increase  in  our  knowledge  of  the  coronal  spectrum 
came  as  the  result  of  the  observations  of  the  Lick  Observa- 
tory L  at  the  Australian  eclipse  of  September  21,  1922.  On 
account  of  the  perfect  weather  conditions  and  long  duration 
of  totality  an  opportunity  was  afforded  for  carrying  out  an 
extensive  program  with  exposures  exceeding  5  minutes. 

The  spectra  taken  with  each  of  the  instruments  of  one- 
prism  dispersion  exhibited  the  continuous  spectrum  and  most 
of  the  bright  lines  confined  to  a  region  4'  to  6'  from  the  sun's 
limb.  The  maximum  was  found  for  the  green  line  with  an 
extension  of  8'  from  the  edge  of  the  sun.  The  spectrum  of 
the  outer  corona  seemed  unquestionably  to  show  the  Fraun- 
hofer lines  which  were  specially  visible  to  the  violet  of  Hy 
where  the  continuous  spectrum  was  less  intense.  No  trace 
of  the  sky  spectrum  was  found  to  exist  beyond  the  limits  of 
the  coronal  spectrum.  Since  the  sky  was  remarkably  clear , 
without  the  slightest  evidence  of  haze  or  clouds,  it  is  manifest 
that  the  Fraunhofer  lines  of  the  1922  eclipse  did  not  take 
their  origin  by  reflection  in  the  earth's  atmosphere.  Moore's 
observations  in  thus  proving  the  Fraunhofer  lines  of  the  outer 
corona  are  caused  by  the  scattering  of  the  sun's  light  by 
some  means  in  the  corona  are  of  the  utmost  importance  in 
advancing  our  knowledge  of  the  perplexing  solar  aureole. 
Comparisons  of  the  measures  of  the  lines  of  the  coronal  and 
sky  spectra  at  two  points  20'  east  and  20'  west  of  the  sun's 
limb  showed  that  the  coronal  lines  on  both  sides  of  the  sun 
were  displaced  to  the  red  of  the  corresponding  sky  lines.  The 
amount  of  displacement  corresponds  to  a  velocity  in  the  line 
of  sight  away  from  the  observer  of  26  km  per  second.  Since 
the  light  observed  was  reflected  sunlight,  the  measured  radial 

1  Moore,  Publ.  A.  S.  P.,  35,  59,  1023. 


384  ECLIPSES  OF  THE  SUN 

velocities  supply  evidence  that  the  particles  of  the  corona 
at  20'  from  the  sun's  edge  are  moving  away  from  the  sun 
with  a  speed  of  the  order  of  20  to  30  km  per  second. 

It  is  highly  probable  that  the  relative  intensities  of  the 
emission  lines  of  the  corona  vary  with  the  sun-spot  period, 
though  information  on  this  point  is  very  meager.  It  is  dif- 
ficult to  compare  spectra  secured  by  different  observers  at 
different  eclipses  using  instruments  of  vastly  differing  re- 
solving powers,  especially  since  the  coronal  lines  are  so  weak 
and  are  seen  projected  on  a  background  of  continuous  spec- 
trum. Under  these  conditions,  the  intensities  of  the  bright 
line  spectrum  secured  with  low  dispersion  instruments  will 
obviously  be  less  intense  than  for  instruments  of  high  dis- 
persion. 

The  spectra  secured  with  the  slit  spectrographs  and  also  by 
means  of  an  instrument  without  slit  seemed  to  prove  conclu- 
sively that  the  coronal  emission  lines  in  1922  were  much 
fainter  than  those  of  the  eclipse  of  1918,  also  obtained  by 
the  Lick-Crocker  expedition.  The  corona  of  1922  was  of 
the  sun-spot  minimum  type.  Hence,  the  suspicions  of  pre- 
vious observers  that  the  bright  lines  of  the  corona  are  fainter 
at  sun-spot  minimum  than  they  are  at  sun-spot  maximum 
seem  to  be  completely  confirmed. 

Various  observers  have  investigated  the  rotation  of  the 
corona  by  measuring  from  spectra  the  Doppler  effect  of  mo- 
tion in  the  line  of  sight.  It  has  been  generally  assumed  that 
the  corona  rotates  with  the  sun,  which  is  at  a  rate  of  2.0  km 
per  second  at  the  limb  of  the  sun.  A  rotational  speed  of 
this  size  corresponds  to  a  shift  in  wave-length  of  the  coro- 
nium  lines  amounting  to  0.035  A.  And  yet,  the  wave-lengths 
for  this  line  in  1918  by  the  Lick  and  Mt.  Wilson  observers, 
each  using  an  efficient  instrument,  differ  by  0.2  A,  or  six 
times  the  rotational  shift!  It  is  probably  no  very  great  ex- 
aggeration to  say  that  at  the  present  time  we  know  abso- 
lutely nothing  regarding  the  rotation  of  the  corona.  It  is 
not  impossible  that  the  wave-lengths  of  the  lines  3388  and 
5303  may  be  determined  with  greatly  increased  accuracy, 
but  these  lines  at  best  are  faint,  and  the  exposures  available 
are  very  short.  The  best  method  of  determining  the  rota- 


THE  CORONA  385 

tional  speed  will  be  unquestionably  by  securing  as  accurate 
wave-lengths  of  these  lines  as  possible,  with  the  slit  of  the 
spectograph  stretching  across  the  sun's  eqtiator.  If  different 
values  of  wave-length  are  obtained  for  east  and  west  limbs, 
thus  indicating  rotation,  the  observer  should  be  very  careful 
that  he  has  eliminated  all  possible  instrumental  sources.  In 
place  of  using  the  solar  spectrum  as  a  comparison,  it  is  prob- 
able that  more  accurate  wave-lengths  will  be  obtained  by 
using  an  artificial  source,  in  the  manner  employed  with 
stellar  spectra.  The  comparison  spectrum  could  readily  be 
superposed  on  the  black  moon. 

Wright  and  Curtis  describe  ]  in  detail  their  unsuccessful 
attempts,  made  at  the  eclipses  of  1923,  1925,  1926  and  1929, 
to  determine  the  rotation  of  the  corona  by  the  method  ap- 
plied to  the  Orion  nebula  by  Buisson,  Fabry  and  Bourget. 

In  spite  of  the  pitifully  small  amount  of  time  available 
for  the  investigation  of  the  spectrum  of  the  corona,  much 
knowledge  has  been  acquired;  but,  unfortunately,  suspicion 
has  also  been  cast  on  some  of  the  information  we  thought 
was  fully  secured.  A  brief  summary  of  what  we  now  think 
we  know  may  not  be  out  of  place.  Three  types  of  spectra 
must  be  distinguished:  the  continuous  spectrum  close  to  the 
edge  of  the  sun,  the  emission  spectrum,  and  the  Fraunhofer 
spectrum.  The  coronal  spectrum  is  now  fully  distinguished 
from  that  of  the  chromosphere.  The  lines  of  calcium  and 
hydrogen,  frequently  photographed  at  mid-totality,  do  not 
belong  to  the  corona,  but  are  caused  by  the  light  of  the 
chromosphere  being  diffused  in  the  earth's  atmosphere, 
usually  by  thin  clouds. 

In  order  to  ascertain  the  atomic  origin  of  coronium,  coro- 
nal spectroscopic  work  needs  more  information  about  (i) 
wave-lengths  of  greater  precision,  and  (2 )  detection  of  simi- 
larities in  structure.  To  attain  the  highest  accuracy  in  wave- 
lengths a  slit  is  desirable.  The  most  useful  form  is  a  three- 
prism  spectrograph  designed  to  gather  as  much  light  as 
possible.  To  secure  a  wide  range  of  wave-lengths,  optical 
parts  of  quartz  are  necessary.  To  detect  similarities  in  the 
structure  of  coronal  lines  slitless  spectra  are  needed.  All 

1  Sprout  Observatory  Publications,  No.  n,  1930. 


386  ECLIPSES  OF  THE  SUN 

coronal  investigations  require  large  dispersion  in  order  to 
increase  the  contrast  between  the  bright-line  coronal  spec- 
trum and  the  continuous  spectrum.  The  great  difficulty  is 
the  pitifully  small  amount  of  light  available  together  with 
the  short  exposures  permitted.  Unfortunately,  a  slit  cannot 
be  used  with  a  concave  grating  —  there  is  not  enough  light. 
Without  a  slit  there  is  little  hope  of  finding  detailed  structure 
in  the  coronal  lines  unless  the  sun  is  in  an  active  condition. 

Out  of  eight  eclipses  observed,  the  author  has  actually  seen 
seven  coronas,  the  1927  phenomenon  being  blotted  out  by 
clouds.  The  corona  that  exhibited  the  most  spectacular 
beauty  and  made  the  most  lasting  impression  was  not  the 
first  eclipse  witnessed,  that  of  1900,  but  the  eclipse  of  1918. 
This  was  the  eclipse  of  pronounced  color,  the  prominences 
were  large  and  brilliant.  The  contrast  between  the  warm, 
rosy  color  of  the  prominences  and  the  pearly-white  filmy 
structure  of  the  corona  left  a  never-to-be-forgotten  impres- 
sion on  the  mind,  that  time  can  never  efface.  On  account  of 
the  greater  number  of  prominences  visible  at  sun-spot  maxi- 
mum, the  eclipse  near  maximum  provides  a  more  wonderful 
picture  to  the  unaided  eye  than  does  the  minimum  type  of 
corona.  The  eclipse  of  1927  therefore  presented  to  those 
fortunate  enough  to  witness  it,  as  gorgeous  a  spectacle  as 
that  of  the  eclipse  of  1842  (see  p.  132  )  which  aroused  both 
the  populace  and  the  astronomers  to  such  a  high  pitch  of 
enthusiasm  that  every  eclipse  from  that  day  to  this  has  been 
assiduously  observed. 

To  observe  and  enjoy  the  beauty  of  the  corona  one  needs 
little  but  his  own  good  eyes.  A  good  pair  of  field  glasses  or 
a  small  telescope  will  permit  the  study  of  the  prominences  or 
of  the  details  of  the  inner  corona.  Baily's  beads,  at  the  be- 
ginning and  at  the  end  of  totality  can  be  more  fully  enjoyed 
by  the  use  of  telescopic  power.  It  should  hardly  be  neces- 
sary to  add  that  one  should  protect  the  eyes  from  the  glare 
of  the  sun  while  the  crescent  sun  is  diminishing  before  the 
advent  of  totality. 


CHAPTER  XXI 

CORONAL  THEORIES 

AS  THE  Fraunhofer  lines  undoubtedly  exist  in  the 
spectrum  of  the  outer  corona,  they  must  be  caused  by 
the  reflection  of  chromospheric  light  by  matter  exist- 
ing in  the  corona  in  a  finely-divided  state.  Fortunately,  there 
are  methods  available  for  testing  the  question  of  scattering 
in  the  corona,  namely,  by  observations  for  the  determination 
of  the  polarization  of  light.  Since  the  eclipse  of  i860,1  when 
Secchi  and  Prazmowski  first  took  up  the  subject,  polariza- 
tion observations  have  found  their  place  at  almost  every 
total  eclipse.  At  the  beginning  of  the  investigations,  they 
were  carried  out  entirely  by  visual  methods;  but  in  a  manner 
similar  to  what  has  happened  in  other  branches  of  astro- 
nomical work,  photographic  observations  have  gradually 
displaced  visual  ones,  and  as  a  consequence  greater  and 
greater  precision  has  been  attained.  The  accurate  determi- 
nation of  the  percentage  of  polarized  light  has  an  important 
bearing  on  the  study  of  the  distribution  of  matter  in  the 
corona.  To  be  of  the  greatest  value,  the  polarization  should 
be  known  for  different  distances  from  the  sun's  limb. 

In  general,  there  are  two  different  methods  of  analyzing 
polarization;  one  is  by  the  use  of  double-image  prisms,  the 
other  by  means  of  plane  mirrors.  There  are  many  varia- 
tions of  these  methods  possible.  In  1900,  Wood  employed 
a  direct  vision  prism  before  the  object-glass  of  a  telescope, 
the  eye-piece  containing  a  Savart  plate  and  a  Nicol  prism. 
This  combination  gave  a  continuous  spectrum  crossed  by 
very  distinct  diagonal  interference  bands,  manifesting  fairly 
strong  polarization  estimated  to  equal  between  10  and  15 
percent.  The  character  of  the  interference  bands  indicated 
that  the  bright-line  spectrum  was  not  polarized,  or  in  other 

1  Comptes  Rendus,  51,  195,  1860. 

387 


388  ECLIPSES  OF  THE  SUN 

words,  that  the  light  causing  these  lines  was  not  reflected 
sunlight.  The  appearance  presented  in  the  telescope  how- 
ever differed  so  materially  l  from  what  had  been  expected 
that  it  took  many  of  the  precious  seconds  of  totality  for  the 
observer  to  readjust  his  ideas;  and  all  the  while  he  could  not 
help  but  feel  that  something  radically  wrong  must  have 
happened  to  the  apparatus.  At  the  same  eclipse,  Dorsey  2 
photographed  the  corona  through  a  double-image  prism  in 
the  manner  utilized  by  A.  W.  Wright  in  1878.  The  method 
gives  two  photographs  of  the  corona  on  each  plate;  one 
having  cut  out  of  it  all  the  light  polarized  along  the  line 
joining  the  two  images,  and  the  other  all  that  polarized  at 
right  angles  to  this  direction.  Hence,  if  the  corona  is 
polarized  radially  or  tangentially,  one  image  will  be  deficient 
in  light  along  the  diameter  perpendicular  to  this  direction. 
Which  image  is  deficient  along  the  line  joining  the  centers 
of  the  two  photographic  images  depends  on  the  kind  of 
double-image  prism  used  and  whether  the  polarization  is 
radial  or  tangential.  Dorsey  also  examined  the  corona 
visually  by  a  polarimeter  consisting  of  a  telescope  in  the 
focal  plane  of  which  was  placed  a  biquartz  half  an  inch 
square.  The  eye-piece  contained  a  Nicol  prism,  and  be- 
tween the  objective  and  the  biquartz  was  a  double  pile  of 
plates.  The  conclusions  were  that  the  corona  is  polarized 
radially,  the  visual  observations  giving  the  amount  of  eleven 
per  cent  at  a  distance  of  8'  from  the  moon's  limb,  a  value 
agreeing  well  with  Wright's  11.2  per  cent  found  at  f  from 
the  moon's  limb  at  the  eclipse  of  1878. 

No  attempt  can  be  made  to  give  here  a  complete  account 
of  the  numerous  observations  made  at  various  eclipses  to 
determine  the  amount  of  polarization.  Special  mention, 
however,  should  be  made  of  the  excellent  work  by  Newall 
and  by  Turner,  each  of  whom  has  observed  polarization 
effects  at  several  eclipses.  In  Lick  Observatory  Bulletin,  6, 
1 66,  1911,  R.  K.  Young  discusses  the  measures  of  photo- 
graphs secured  by  Perrine  of  the  Lick  Observatory  at 
eclipses  of  1901,  1905  and  1908.  At  the  Sumatra  eclipse, 

1  Publications  of  the  U.  S.  Naval  Observatory,  4,  D   116,  IQO$. 

2  Publications  of  the  U.  S.  Naval  Observatory,  4,  D  117,  1905. 


THE  CORONA  PHOTOGRAPHED  WITH  POLARIZED  LIGHT.     LICK  OBSERVATORY 

EXPEDITION,  JANUARY  3,   1908 
Note  the  difference  in  the  distribution  of  coronal  light  in  the  two  photographs. 


N 


CURVES  SHOWING  INTENSITY  OF  LIGHT  IN  CORONA,  AUGUST  21 ,  1914 
Measures  by  Bcrgstrand  in  units  of  stellar  magnitude. 


CORONAL  THEORIES  389 

the  plates  were  secured  by  a  double-image  camera  with  the 
prism  set  in  succession  at  five  different  positions  separated 
by  angles  of  a  quarter  of  a  right  angle.  At  the  two  succeed- 
ing eclipses,  photographs  were  secured  by  the  same  double- 
image  camera  and  also  by  a  reflecting  polarigraph.  This 
latter  consisted  of  three  cameras  with  lenses  of  three  inches 
aperture  and  fifty  inches  focal  length.  In  front  of  each  of 
two  of  the  lenses  was  placed  a  glass  reflector,  so  inclined 
that  the  light  from  the  corona  was  incident  at  the  polarizing 
angle,  the  planes  of  polarization  of  the  two  reflectors  being 
perpendicular  to  each  other.  The  third  camera  was  used 
merely  as  a  check.  The  measures  showed  that  the  polariza- 
tion was  radial  and  that  the  percentage  of  observed  light 
increased  rapidly  from  the  limb,  reaching  a  maximum  of 
thirty-seven  percent  at  5'  distance,  and  then  diminished 
slowly,  being  thirty-five  percent  at  9'  from  the  limb.  As- 
suming the  well-known  law  of  the  reflection  and  scattering 
of  light  that  it  varies  inversely  as  the  fourth  power  of  the 
wave-length,  the  value  of  u  per  cent  in  the  visual  re- 
gion 5600  A  would  correspond  to  33  per  cent  in  the  photo- 
graphic region  4270  A.  A  close  accord  is  thus  seen  to  exist 
between  the  visual  values  obtained  from  the  eclipses  be- 
fore 1901,  and  the  photographic  results  from  the  three 
eclipses  of  1901,  1905  and  1908.  In  view  of  the  very 
great  difference  between  the  amounts  of  polarization  in  the 
visual  and  photographic  regions,  it  is  highly  desirable  that 
values  in  the  visual  region  be  obtained  by  photographic 
methods  by  the  use  of  a  color  filter  and  isochromatic  plates. 
In  view  of  the  experience  of  Wood  at  the  eclipse  of  1900, 
it  is  urged  that  all  observations  in  the  future  for  polar- 
ization effects  be  made  photographically.  Observations 
might  possibly  still  be  made  visually  by  experienced 
observers,  but  such  values  should  be  looked  upon  merely 
as  checks  on  the  more  accurate  photographic  results.  Ob- 
servers should  be  most  careful  to  know  the  amount  of  polar- 
ization caused  by  the  apparatus  itself  so  as  to  eliminate 
these  effects  from  the  total  polarization  observed  during 
the  progress  of  the  total  eclipse.  As  is  well  known/  every 

1  Wood,  Astrophyskal  Journal,  12,  283,  1900. 


390  ECLIPSES  OF  THE  SUN 

form  of  apparatus  that  disperses  light,  at  the  same  time  po- 
larizes it.  A  Rowland  grating  gives  strongly  polarized  spec- 
tra; and  with  prismatic  spectra,  as  the  dispersion  is  in- 
creased by  additional  prisms,  the  polarization  is  likewise 
increased  by  the  new  surfaces  added. 

At  the  eclipse  of  1918,  Lewis  of  the  Lick  Observatory 
party,  by  means  of  two  separate  double-image  cameras,  se- 
cured successful  photographs  in  two  different  regions  of  the 
spectrum  by  using  blue  and  green  color  filters.  The  effect 
for  the  blue  was  found  to  be  greater  than  for  the  green. 
Quantitative  values  for  the  amount  of  polarization  could  not 
be  furnished  however  for  two  reasons:  first,  the  law  of 
diminution  of  the  intensity  of  coronal  radiation  at  different 
distances  out  from  the  moon's  limb  is  unknown;  and  second, 
the  effect  on  the  corona,  of  polarization  of  the  light  of  the 
sky  surrounding  the  corona,  has  not  been  fully  investigated. 
At  the  eclipse  of  1905,  Newall  '  found  that  at  a  distance  of 
three-quarters  of  a  degree  from  the  center  of  the  corona, 
the  strength  of  the  Savart  bands  from  the  sky  neutralized 
those  from  the  corona.  This  signifies  that  from  the  veil  of 
the  illuminated  sky  between  the  observer  and  the  corona 
there  came  as  much  polarized  light  as  from  the  corona  three- 
quarters  of  a  degree  from  the  center.  Unquestionably,  the 
character  and  intensity  of  the  atmospheric  polarization  vary 
considerably  at  different  eclipses,  which  of  necessity  are 
observed  under  different  conditions  of  clouds  and  moisture 
in  the  terrestrial  atmosphere.  On  account  of  the  great  in- 
tensity of  the  corona  close  to  the  sun,  for  instance,  at  i' 
from  the  limb,  it  is  difficult  to  measure  the  intensity  of  the 
darkening  of  the  photographs  and  hence  to  evaluate  the 
amount  of  polarization  so  close  to  the  edge  of  the  sun. 
Newall  in  1901  obtained  "  quite  marked  polarization  "  at 
i'  from  the  limb.  The  Savart  photographs  for  testing  po- 
larization seem  to  possess  some  advantages  over  the  double- 
image  or  reflection  methods  (see  Newall,  loc.  cit.). 

The  only  means  of  unravelling  some  of  these  puzzles 
seems  to  lie  in  determining  the  law  of  change  in  the  intensity 
of  the  corona  at  different  distances  out  from  the  limb  of 

i  Monthly  Notices,  R.  A.  S.,  66,  475,  1906. 


CORONAL  THEORIES  391 

the  sun.  The  law  best  known  is  that  of  Turner,1  as  the  re- 
sult of  photographs  obtained  in  1898,  that  the  intensity  of 
the  corona  varies  from  the  edge  of  the  sun  outwards  in- 
versely as  the  sixth  power  of  the  distance  measured  from  the 
center  of  the  sun.  At  the  eclipse  of  1905,  Schwarzschild L> 
confirmed  Turner's  law,  and  this  same  eclipse,  Graff d  as- 
sumed the  correctness  of  this  law  to  determine  the  law  of 
blackening  of  his  photographic  plates.  But  at  this  same 
eclipse  of  1905,  Becker4  found  the  intensity  of  the  corona 
subject  to  a  different  law,  that  it  varied  inversely  as  the 
fourth  power  of  the  distance  counted  from  a  point  one- 
seventh  of  a  solar  radius  inside  the  edge  of  the  sun.  At  the 
eclipse  of  1908  by  means  of  measures  carried  out  by  the 
bolometer,  Abbot r>  confirmed  Becker's  law  rather  than  that 
of  Turner,  while  R.  K.  Young  (loc.  cit.)  found  an  intensity 
depending  on  the  inverse  sixth  and  eighth  powers  of  the 
distance  measured  from  the  center  of  the  sun. 

A  very  different  law  was  found  by  Bergstrand  in  a  very 
important  publication  entitled  "  Etudes  sur  la  distribution 
de  la  lumiere  dans  la  couronne  solaire,"  Upsala,  1919.  From 
photographs  secured  at  the  eclipse  of  August  21,  1914,  an 
attempt  was  made  to  determine  the  relative  intensity  of 
light  distributed  within  the  corona.  The  measurement  of 
the  absolute  intensity  and  the  estimation  of  the  total  light 
of  the  corona,  compared  for  instance  with  that  of  the  full 
moon,  did  not  form  part  of  the  program.  The  problem  is 
one  of  photometry,  and  for  its  solution  can  be  brought  the 
vast  experience  gained  by  many  years  of  investigation  in 
determining  the  magnitudes  of  the  stars.  Of  the  several 
methods  available,  Bergstrand  adopted  the  plan  of  employ- 
ing twin  photographic  objectives,  mounted  equatorially  in 
such  a  manner  that  the  two  solar  images  could  be  impressed 
upon  one  and  the  same  photographic  plate.  On  the  day  of 
the  eclipse  the  times  of  exposure  of  the  two  objectives  were 

1  Popular  Astronomy,  14,  548,  1906. 

2  Astron.  Mitteil.  zu  Gottingen,  13,  iqo6. 

3  Astron.  Abhandl.  der  Hamburger  Sternw.  in  Bergedorf,  j,  i,  1913. 

4  Memoirs,  R.  A.  S.,  57  and  Phil.  Trans.  Roy.  Soc.  207  A,  1908. 
6  The  Sun,  133,  1911. 


392  ECLIPSES  OF  THE  SUN 

made  identical,  but  the  aperture  of  one  of  the  objectives 
was  reduced  by  means  of  a  suitable  diaphragm  to  one-third 
that  of  the  other. 

The  intensity  of  the  silver  deposit  measured  on  the  plates 
is  the  summation  of  two  separate  effects,  one  of  which  is 
due  to  the  corona  itself  while  the  other  comes  from  the  dif- 
fuse light  of  the  sky.  Added  to  these  two,  there  is  in  reality 
a  third  effect  found  close  to  the  moon's  limb,  that  of  a  halo 
caused  by  reflection  from  the  glass-side  of  the  plate  of  the 
strong  illumination  of  the  inner  corona.  Fortunately,  the 
intensity  of  the  corona  could  be  separated  from  the  two 
other  effects.  The  values  thus  secured  do  not  in  any  man- 
ner confirm  Turner's  law  of  the  inverse  sixth  power  nor  yet 
the  law  of  Becker  according  to  which  the  intensity  varies  in- 
versely proportional  to  the  fourth  power.  In  fact,  Bergstrand 
finds  that  the  intensities  near  the  solar  equator  differ  greatly 
from  those  near  the  poles,  the  equatorial  rays  having  an  in- 
tensity three  times  as  great  as  the  polar  rays.  The  equa- 
torial and  polar  intensities,  however,  can  be  brought  into 
relationship  with  each  other  in  a  very  simple  manner  by 
supposing  that  the  corona  is  composed  of  two  phenomena. 
One  of  them,  the  "  interior  corona  "  exists  exclusively  in  the 
equatorial  zone.  In  both  of  these  phenomena,  the  intensity 
of  the  light  is  inversely  proportional  to  the  square  of  the 
distance  measured  from  the  edge  of  the  sun,  the  intensity 
of  the  "  equatorial  corona  "  being,  however,  about  double 
that  of  the  "  interior  corona."  On  page  389  is  given  a  curve 
representing  Bergstrand Js  values.  Measures  were  carried 
out  on  solar  radii  separated  from  each  other  by  15°.  Posi- 
tion angles  are  designated  from  the  north  towards  the  east. 
The  intensity  at  a  distance  of  one  radius  from  the  edge  of 
the  sun  is  taken  as  unity,  and  values  are  represented  in  terms 
of  stellar  magnitudes,  where  a  difference  of  five  magnitudes 
represents  a  change  of  a  hundred-fold  intensity.  The 
strongest  coronal  rays  at  times  depart  sensibly  from  the 
direction  of  the  solar  radius.  This  is  shown  by  a  jet  which 
leaves  the  sun  at  position  angle  25°  but  which  does  not  go 
out  radially  and  is  found,  in  the  external  curves  between 
30°  and  45°.  Some  of  the  most  intense  rays  apparently 


CORONAL  THEORIES  393 

do  not  take  their  origin  from  the  edge  of  the  sun  but  rather 
from  the  front  or  back  side  of  the  solar  disk.  Moreover, 
the  structure  of  the  corona  is  highly  complicated,  since  the 
distribution  of  jets  is  not  uniformly  distributed  in  all  longi- 
tudes and  since  they  frequently  depart  sensibly  from  the 
radial  direction.  On  account  of  the  greater  strength  of  the 
equatorial  rays,  it  was  possible  for  Bergstrand  to  observe 
these  rays  on  the  photographic  plates  to  a  distance  of  ten 
radii,  or  five  solar  diameters  from  the  edge  of  the  sun,  before 
they  diminish  in  intensity  to  that  of  the  diffuse  sky  light.  In 
the  polar  direction,  equality  was  attained  at  a  distance  of 
three  and  a  half  solar  diameters. 

Valuable  observations  were  secured  by  photography  at  the 
1925  eclipse1  by  Pettit  and  Nicholson.  They  used  a  Ross 
6-inch  doublet  of  1 5-foot  focus  fed  by  a  coelostat.  Exposures 
were  made  in  photographic  light  and  also  at  wave-lengths 
greater  than  6100  A.  Standard  photometric  squares  were 
impressed  on  each  plate.  The  plates  were  mounted  on  a 
turntable  and  measures  were  made  by  the  Koch  micropho- 
tometer.  Two  corrections  were  applied  to  the  measures,  one 
on  account  of  general  scattered  sky  light,  when  it  was  as- 
sumed that  the  mean  density  of  the  plate  at  the  four  cor- 
ners gave  a  measure  of  the  sky  illumination.  The  other  cor- 
rection was  for  halation  and  scattered  coronal  light.  No 
correction  was  applied  for  the  polarization  at  the  coelostat 
mirror.  Pettit  and  Nicholson  find  that  their  measures  in  the 
photographic  region  confirm  the  inverse  sixth  power  from  the 
center  of  the  sun,  while  those  in  the  visual  region  obey  the  in- 
verse seventh  power.  However,  the  measured  values  both 
in  the  photographic  and  visual  regions  more  closely  con- 
form "'  to  the  inverse  fourth  power  measured  from  a  point  not 
coinciding  with  the  center  of  the  sun.  The  1925  photographs 
were  measured  to  a  distance  of  three  radii  from  the  sun's 
center  while  Bergstrand 's  1914  photographs  were  measured 
to  ten  radii.  Also  at  the  1925  eclipse,  King  and  Miss  Har- 
wood 8  found  that  the  intensity  of  coronal  radiation  at  dis- 

1  Astrophysical  Journal,  62,  202,  1925. 

2  Handbuch  der  Astro  physik,  4,  333,  1929. 

3  Harvard  Circulars,  312,  1927. 


394  ECLIPSES  OF  THE  SUN 

tances  from  the  sun's  limb  greater  than  io'.$  follows  the  law 
of  inverse  squares. 

Following  the  eclipse  of  1926,  Stetson  and  Andrews  !  dis- 
cuss the  measures  made  by  different  observers  (including 
themselves)  and  come  to  the  conclusion  that  no  single  law 
can  express  the  brightness  of  the  solar  corona  as  a  simple 
function  of  the  distances  either  from  the  limb  or  from  the 
sun's  center.  Within  2%  radii  of  the  center,  the  intensity 
appears  to  decrease  as  the  seventh  power,  from  2-\  to  3  radii 
the  inverse  fourth  power  fits  better,  while  further  out  the  law 
is  the  inverse  square. 

At  the  1927  eclipse,  Belanovsky  and  Perepelkin  J  secured 
photographs  through  thin  clouds  which  were  measured  by 
the  Hartmann  microphotometer.  From  the  measures,  lines 
of  equal  intensities,  or  isophotes,  were  drawn  which  showed 
that  the  corona  decreased  in  intensity  according  to  power  2.7 
with  respect  to  distances  taken  near  the  edge  of  the  sun. 

At  the  1929  eclipse,  the  German  expedition  secured  '  ex- 
cellent photographs  with  a  horizontal  camera  of  28-foot  focus 
and  with  an  astrographic  telescope  of  i  i-foot  focus.  A  thor- 
ough discussion  has  been  made  by  von  Kliiber.  The  photo- 
graphs of  both  cameras  were  measured  by  the  Hartmann 
microphotometer  and  isophotes  were  drawn  giving  lines  of 
equal  intensity  in  the  corona.  The  astrographic  plates  show 
that  the  corona  decreases  in  intensity  according  to  power  2.5 
in  units  of  distances  from  the  edge  of  the  sun.  With  the  hori- 
zontal camera,  at  the  sun's  poles  the  plates  show  that  the 
decrease  in  coronal  intensity  is  according  to  the  power  2.0, 
while  at  the  equator  the  exponent  2 .3  more  correctly  repre- 
sents the  observations.  All  of  the  1929  measures  together 
are  best  represented  by  the  inverse  power  2 .4  times  the  dis- 
tance from  the  sun's  edge. 

It  is  evident  that  our  knowledge  regarding  the  distribu- 
tion of  light  within  the  corona  is  in  a  very  unsatisfactory 
state,  since  the  law  of  the  intensity  has  been  found  to  be 
inversely  as  the  second,  fourth,  sixth  or  even  eighth  power 

1  Astro  physical  Journal,  6g,  227,  1925 

-  Monthly  Notices,  R.  A.  S.,  88,  740,  1928. 

>A  Zeit.  fur  Astrophysik,  2,  289;  3,  142,  1931. 


CORONAL  THEORIES  ,'395 

of  the  distance  from  the  sun.  It  is  consequently  of  great 
importance  that  a  well-devised  form  of  apparatus  be  con- 
structed for  use  at  eclipses,  and  that  photographs  be  secured 
both  in  the  violet  and  visual  regions  on  a  carefully  prepared 
plan  at  several  future  eclipses.  The  apparatus  used  by 
Bergstrand  seems  to  leave  little  room  for  improvement. 
If  the  photographic  plates  secured  at  the  eclipse  could  be 
impressed  by  light  from  a  standard  source,  and  if  in  addition 
photographs  of  the  full  moon  were  obtained,  we  should  then 
be  in  a  position  of  having  information  additional  to  that 
acquired  during  the  progress  of  an  eclipse.  We  need  to 
know  whether  the  intensity  of  the  distribution  of  light 
within  the  corona  follows  the  same  law  at  every  eclipse,  or 
whether  this  law  varies  according  to  the  sun-spot  period, 
and  we  need  to  know  the  law  both  in  the  blue  and  yellow 
regions.  Many  of  the  coronas  have  been  observed  through 
clouds  or  haze,  or  with  varying  conditions  of  transparency. 
Unfortunately  it  has  not  been  possible  to  make  proper  al- 
lowances for  these  varying  factors  with  the  result  that  the 
observations  are  undoubtedly  affected  by  systematic  errors. 
Not  until  we  secure  more  and  better  standardized  observa- 
tions can  we  expect  to  advance  much  in  the  solution  of  coro- 
nal problems. 

In  considering  the  measurements  made  for  determining 
the  total  light  of  the  corona,  we  see  similar  evidences  of  large 
systematic  errors  depending  on  transparency  conditions  at 
the  time  of  the  eclipse.  The  following  table  is  taken  from 
Handbuch  der  Astrophysik,  4,  336,  1929,  and  brought  up  to 
date.  The  total  light  of  the  corona  is  expressed  in  terms  of 
the  total  light  from  the  full  moon. 

The  photographic  determinations  at  the  eclipse  of  1886 
and  the  two  eclipses  in  1889  give  values  that  are  apparently 
too  small,  while  one  value  in  1898  is  too  large.  Giving  half 
weight  to  each  of  these  four  determinations,  the  weighted 
mean  shows  the  total  light  of  the  corona  to  possess  55  per 
cent  of  the  light  of  the  full  moon.  Results  from  one  ex- 
pedition may  be  given  here.  Kunz  and  Stebbins  observed 
the  1918  eclipse  with  a  potassium  photo-electric  cell.  They 
compared  the  light  of  the  corona  with  a  standard  candle, 


396 


ECLIPSES  OF  THE  SUN 


TOTU.  LIGHT  OF  THE  CORONA 


Eclipse 

Method 

Observer 

In  terms  of 
full  moon 

1886 

Photographic 

W,  H.  Pickering 

0  025 

1889,  January 

Holdcn 

o  04 

1889,  December 

Holden 

0   02 

i893 

Turner 

o  b 

1898 

Turner 

r    i 

1898 

Bacon  and  Gare 

2  7 

1905 

Graff 

O    20 

1905 

Schwarzschild 

o  17 

1908 

Pcrrinc 

0    TI 

1886 

Visual 

Abney  and  Thorpe 

o  8 

1889,  January 

Leuschner 

o  4 

i893 

Abney  and  Thorpe 

I    1 

•  1905 

Fabry 

o  75 

1905 

Knopf 

o  85 

1908 

Bolometer 

Abbot 

o  20 

1918 

Photo-Electric 

Kunz  and  Stcbbins 

o  50 

1922 

a 

Briggs 

o  41 

1925 

Microphotometer 

Parkhurst 

o  27 

1925 

Thermocouple 

Petti  t  and  Nicholson 

o  52 

1926 

Thermopile 

Stetson  and  Coblentz 

o  52 

with  two  electric  lamps,  with  the  full  moon,  and  with  an  area 
of  the  sky  during  totality  and  during  full  sunshine.  Their 
numerical  results  are  as  follows: 

Observed  total  light  of  the  corona  0.60     candle-meters 
Same,  corrected  to  outride  of  atmosphere  i  07     candle-meters 
Observed  ratio  of  corona  to  full  moon  o  6 
Same,  corrected  to  outside  of  atmosphere  0.50 
Observed  ratio  of  corona  to  sky  circle  of  diam- 
eter one-half  degree  and  8°  from  uneclipsed  sun        0.105 
Same  during  totality  640. 

The  photo-electric  cell  with  which  these  measures  were 
secured  had  a  maximum  sensibility  at  wave-length  4500  A 
in  the  blue. 

Further  details  about  the  earlier  eclipses  may  be  obtained 
from  Handbuch  der  Astrophysik.  In  order  to  illustrate  the 
inconsistencies  in  the  best  work  along  these  lines,  a  few 
words  may  be  said  about  the  more  recent  eclipses,  begin- 
ning with  1925.  At  this  eclipse,  Harvard  Observatory  occu- 
pied four  stations  in  order  to  measure  the  total  light  of  the 
corona  by  a  photometer  of  "  pinhole "  type  designed  by 


CORONAL  THEORIES  397 

King.  It  was  found  that  the  integrated  brightness  within 
a  circle  3°  in  diameter,  given  in  stellar  magnitudes,  is: 
photographic  -10.96,  photovisual  — 11.61;  within  a  cir- 
cle 6°  in  diameter  the  values  are  -11.40  and  -11.71,  re- 
spectively. After  eliminating  the  effect  of  the  illuminated 
sky,  the  stellar  magnitude  of  the  corona  is:  photographic 
-  10.76,  photovisual  -  11.57.  With  one  of  these  photome- 
ters at  the  1926  eclipse,  Stetson  and  his  co-workers  ob- 
tained conflicting  results  in  the  circles  3°  and  6°  in  diameter; 
but  the  measures  seemed  to  show  that  the  corona  was  40 
per  cent  brighter  in  1926  than  in  1925. 

It  is  interesting  also  to  compare  the  measures  of  the  total 
illumination  of  corona  plus  sky.  In  1925  the  horizontal  illu- 
mination was  0.24  foot-candles,  or  equal  to  the  total  in- 
tensity 30  minutes  after  sunset;  in  1926,  measures  by  the 
Macbeth  illuminometer  gave  0.14  foot-candles;  in  1929,  a 
value  0.15  and  1930  (Niuafoou  Island)  0.38  foot-candles. 
The  measures  on  the  three  last  eclipses  were  carried  out  by 
Stetson's  instruments.  The  above  measures  seem  to  show 
that  although  the  1926  corona  was  40  per  cent  brighter  than 
that  of  1925,  the  total  illumination  of  corona  and  sky  to- 
gether was  40  per  cent  fainter. 

Another  instance  of  the  uncertainties  underlying  meas- 
ures of  coronal  radiation  was  shown  at  the  1926  eclipse. 
With  the  photographic  photometer  it  was  found  that  the 
total  brightness  of  the  corona  was  but  one-tenth  that  of 
corona  plus  sky  as  measured  with  the  illuminometer. 

At  the  eclipse  of  1918,  Aldrich1  found  that  the  total 
brightness  of  the  sky  during  totality  was  less  than  that  of 
twilight  one  hour  after  sunset  of  the  same  day.  At  the 
Australian  eclipse  of  1922,  Ross  directed  a  camera,  from 
which  the  lenses  had  been  removed,  toward  the  south 
celestial  pole  and  exposed  a  photographic  plate  during  to- 
tality. Other  plates  from  the  same  box  were  exposed  on 
the  evening  of  the  same  day  for  equal  intervals  of  time  at 
6:14,  6:17,  6:20,  6:23  and  6:26  by  the  clock.  The  central 
portions  of  the  plate  were  cut  out  and  all  six  plates  were 
developed  together.  The  plates  showed  a  regular  gradation, 

1  Smithsonian  Miscellaneous  Collections,  60,  No.  9,  1919. 


398  ECLIPSES  OF  THE  SUN 

It  was  found  that  the  illumination  at  the  south  celestial  pole 
corresponded  with  that  when  the  sun's  center  was  97°  29' 
from  the  zenith. 

It  is  evident  from  the  above  summaries  of  measures  that 
our  present  knowledge  regarding  the  total  amount  of  light 
received  from  the  corona  is  in  a  very  unsatisfactory  state. 
Photometric  methods  present  many  complications.  When 
results  at  one  eclipse  are  compared  with  those  of  another, 
the  instruments  must  be  thoroughly  calibrated  and  methods 
properly  standardized  before  we  can  have  any  confidence  in 
the  conclusions.  To  obtain  the  total  light  of  the  corona,  it 
is  necessary  to  extrapolate  to  the  edge  of  the  moon  and  even 
to  the  edge  of  the  sun.  Unfortunately  we  seem  to  have  little 
sound  information  about  the  law  of  intensity  in  the  corona 
close  to  the  sun's  edge.  The  greatest  difficulty  of  all  is  that 
the  corona  perforce  must  be  observed  through  clouds  and 
haze  at  half  the  eclipses  where  the  corona  is  at  all  visible 
and  through  varying  transparencies  at  the  other  half.  Up 
to  the  present  we  know  of  no  adequate  methods  of  making 
proper  allowance  for  varying  atmospheric  conditions.  Tak- 
ing everything  into  consideration,  it  seems  highly  probable 
that  the  accidental  and  systematic  errors  existing  in  meas- 
ures of  coronal  radiation  certainly  amount  to  20  per  cent, 
and  possibly  50  per  cent,  of  the  final  values.  Roughly 
speaking,  the  total  light  of  the  corona  is  one-half  that  of  the 
full  moon  and  one-millionth  that  of  the  noon-day  sun. 

Hence  we  must  not  take  too  seriously  the  attempts  to  cor- 
relate intensity  of  coronal  radiation  with  sun-spot  activity. 
It  will  be  necessary  to  await  more  accurate  observations  of 
the  future.  It  seems  entirely  probable  that  the  inner  corona 
at  sun-spot  maximum  must  be  brighter  than  at  sun-spot 
minimum.  Moreover,  as  the  inner  corona  contributes  the 
most  energy  to  the  total  coronal  radiation  we  would  there- 
fore logically  expect  that  the  total  energy  at  maximum  of 
spots  is  greater  than  at  minimum.  But  to  confirm  this  from 
observations  already  secured  is  another  story.  When  we 
consider  the  total  amount  of  time  available  for  coronal  in- 
vestigations, we  must  not  be  too  discouraged  with  the  re- 
sults obtained  to  date. 


CORONAL  THEORIES  399 

In  former  chapters  it  has  been  shown  that  for  half  a 
century  it  has  been  recognized  that  there  is  a  close  connec- 
tion between  the  shape  of  the  corona  and  the  sun-spot  cycle. 
At.  minimum  of  spots  the  corona  shows  the  long  equatorial 
extensions  and  the  strong  polar  brushes,  while  at  spot  maxi- 
mum the  corona  is  more  nearly  circular  in  outline.  The  first 
to  call  attention  to  this  was  Ranyard  in  1879  in  the  "  Eclipse 
Volume  "  of  the  Royal  Astronomical  Society.  In  1897,  Han- 
sky  made  the  connection  more  certain  by  publishing  a  series 
of  reproductions  of  the  corona  arranged  according  to  the 
sun-spot  curve. 

In  Handbuch  der  Astrophysik,  4,  317,  1929,  there  is  stated 
the  general  problem.  For  the  past  forty  years,  since  accu- 
rate photographs  became  available,  the  dates  of  maxima  and 
minima  of  spots  have  been  as  follows: 

Maximum  Minimum 

1893.6 

1901.7 
1905.6 

1913.6 
1917.6 

1923,6 
1928.5 

The  eclipses  of  the  past  decade  have  been  beautifully 
situated  with  respect  to  the  spot  cycle.  The  eclipse  of  1922 
took  place  shortly  before  minimum  of  spots,  that  of  1923 
almost  exactly  at  minimum.  A  year  and  a  half  after  mini- 
mum came  the  1925  eclipse,  while  those  of  1926  and  1927 
were  just  before  and  1929  and  1930  just  after  maximum. 

The  information  on  the  activity  of  the  sun,  as  recorded 
in  the  sun-spot  cycle,  comes,  as  the  name  signifies,  from 
observations  on  spots,  their  Zurich  relative  numbers,  the 
Greenwich  means  areas,  the  latitude  of  spots,  etc.  Similarly 
observations  made  on  prominences,  give  also  the  number, 
mean  areas,  mean  latitude,  etc.  The  sun-spot  and  prominence 
curves  closely  parallel  each  other.  In  a  sense,  the  informa- 
tion from  prominences  supplements  that  from  spots  in  that 
the  spots  are  phenomena  observed  on  the  face  of  the  sun  while 
the  prominences  are  photographed  only  at  the  sun's  limb. 


400  ECXIPSES  OF  THE  SUN 

To  observe  spots  a  moderate  equipment,  merely  a  tele- 
scope, is  all  that  is  necessary.  The  activity  of  the  sun 
manifesting  itself  in  spots  can  be  observed  as  the  spot  moves 
across  the  face  of  the  sun;  measurements  of  polarity,  etc., 
continued  from  day  to  day,  give  a  fairly  faithful  indication  of 
the  relative  activity  of  the  sun.  In  comparison,  the  promi- 
nences are  transitory  phenomena.  Even  if  individual  promi- 
nences were  more  permanent  than  they  are,  the  rotation  of 
the  sun  would  carry  them  quickly  out  of  sight.  Occasionally 
when  a  total  eclipse  comes,  the  direct  photographs  of  promi- 
nences and  the  eclipse  spectra  can  be  compared  with  spec- 
troheliograph  photographs.  Then  we  see  the  limitations  of 
the  latter  method.  The  eclipse  photographs  on  October  21, 
1930,  showed  the  sun  with  a  very  stormy  region  in  the  south- 
east quadrant,  while  the  Mt.  Wilson  spectroheliogram  taken 
only  a  few  hours  earlier  showed  nothing  particularly  re- 
markable about  the  activity  in  this  quadrant.  Unless  the 
prominence  exhibits  both  height  and  contrast,  through  masses 
of  gases  in  eruption,  it  is  usually  not  a  conspicuous  object 
on  the  spectroheliograph  plates.  Hence,  all  things  consid- 
ered, it  has  been  generally  felt  by  the  average  astrono- 
mer that  the  information  from  spots  gives  a  more  reliable 
indication  of  the  activity  of  the  sun  than  is  obtainable 
from  observations  of  prominences.  As  already  stated, 
however,  the  two  types  of  observation  supplement  each 
other. 

When  attempts  are  made  to  find  correlations  between 
coronal  disturbances  and  either  spots  or  prominences,  it  is 
evident  that  many  more  connections  must  always  be  found 
between  prominences  and  coronal  structure  than  between 
spots  and  corona.  The  spots  are  seen  on  the  face  of  the  sun 
while  both  the  corona  and  prominences  stretch  out  from  the 
edge  of  the  sun.  It  must  not  be  thought  that  there  must  be 
a  more  intimate  connection  between  prominences  and  coronal 
activity  than  there  is  between  sun-spots  and  coronal  disturb- 
ances. The  eclipse  photographs  show  prominences  and  dis- 
turbed regions  in  the  corona  but  from  the  nature  of  things 
cannot  show  spots.  Unless  the  eclipse  astronomer  observes 
the  spots  on  the  final  days  before  the  eclipse — and  he  is 


THE  INNER  CORONA  IN  1926 
Photographed  with  the  63 -foot  tower  telescope  by  the  Swarthmore  College  Expedition. 


CORONAL  THEORIES  401 

usually  too  busy  with  a  thousand  and  one  preparations  that 
must  be  made  —  or  unless  he  looks  up  the  literature  after- 
wards (and  this  is  rarely  done),  the  possible  connections 
between  individual  spots  and  coronal  disturbances  pass  un- 
noticed. 

Ludendorff  has  done  a  valuable  piece  of  work  in  providing 
a  simple  method  of  measuring  the  shape  of  the  corona.  He 
had  the  happy  inspiration  of  utilizing  the  published  photo- 
graphs and  half-tone  reproductions  to  draw  roughly  by  eye 
lines  of  equal  intensities  (isophotes)  in  the  corona.  In  some 
instances,  isophotes  were  already  available  in  the  original 
sources,  especially  in  the  corona  of  1905  from  the  investi- 
gations of  Graff  and  in  that  of  1914  from  the  hands  of 
Bergstrand.  Comparisons  of  the  results  from  Ludendorff 's 
tracings  with  the  more  accurate  methods  of  Graff  and  Berg- 
strand  showed  no  systematic  differences.  From  all  of  the 
tracings  of  each  eclipse,  two  quantities  a  and  b,  were  deter- 
mined by  the  method  of  least  squares.  The  value  a  is  the 
ellipticity  of  the  corona  at  the  edge  of  the  sun,  while  a  +  b 
is  the  ellipticity  at  a  distance  of  one  radius  out  from  the 
sun's  edge.  From  the  examination  of  thirteen  eclipses  be- 
tween 1893  and  1927,  Ludendorff  found  that  the  quantity  a 
was  nearly  constant  for  all  eclipses.  On  the  contrary,  the 
value  b  varied.  For  the  coronas  near  sun-spot  maximum, 
the  value  of  b  is  very  nearly  zero,  the  corona  being  approxi- 
mately circular  in  outline.  Near  minimum  of  spots,  how- 
ever, b  has  increased  in  value,  the  corona  being  more 
elliptical. 

Mitchell  has  found  a  closer  connection  between  the  coro- 
nal shape  and  sun-spot  numbers  than  there  is  with  the 
phase  in  the  spot  cycle  measured  from  the  time  elapsed 
from  minimum  or  maximum  of  spots.  The  table  in  Hand- 
buck  der  Astrophysik,  4,  338,  1929,  is  here  extended  to  in- 
clude the  eclipses  of  1922,  1929,  and  1930.  The  values  from 
the  1929  eclipse  are  from  isophotes  discussed  by  von  Kliiber 
(loc.  cit.}.  Photographs  of  the  1922  eclipse  were  kindly  sent 
me  by  Moore,  and  of  the  October,  1930  eclipse  by  Marriott. 
The  measures  of  both  eclipses  (following  Ludendorff 's 
methods)  were  made  by  Miss  Williams. 


402 


ECLIPSES  OF  THE  SUN 


SPOTS  DECREASING 

SPOTS  INCREASING 

Eclipse 

Spot 
Numbers 

a 

b 

Eclipse 

Spot 
Numbers 

a 

i  $93  3 

88 

o  03 

0 

1926  o 

77 

o  06 

o 

igi8  4 

69 

o  13 

O    IO 

1927  5 

67 

o  04 

o 

1929    3 

59 

0    10 

0   02 

1905  7 

59 

0   01 

o 

1908  o 

45 

o  06 

O   O2 

1030  8 

35 

o  04 

o  23 

i8g8  i 

27 

o  06 

O    12 

1896  6 

22 

o  03 

o  23 

1901  4 

1  1 

o  04 

o  26 

1923  7 

n 

o  06 

0 

IQOO  4 

9 

o  03 

o  29 

1914  6 

9 

o  05 

0 

1925   i 

8 

o  05 

0 

1922  7 

6 

o  04 

o  24 

o  03* 
or 
oo 
oo 


18 
16 
10 


*  All  values  of  a  and  6  are  positive  except  b  of  the  1893  eclipse 

On  the  left  side  of  the  table  are  the  eclipses  between 
maximum  and  minimum  of  spots,  or  on  the  descending 
branch  of  the  sun-spot  curve,  while  on  the  right  hand  are 
the  eclipses  after  minimum  with  spots  increasing  in  numbers. 
One  should  read  down  on  the  left  and  then  up  on  the  right 
side  with  spots  increasing.  Instead  of  arranging  the  ma- 
terial according  to  the  phase  in  the  spot  cycle,  as  was  done 
by  Ludendorff,  it  has  been  arranged  according  to  the  mean 
of  the  spot  numbers  in  a  synodic  month  of  the  sun's  rota- 
tion, i.e.,  the  mean  of  13  days  before  and  after  the  eclipse, 
also  the  day  of  the  eclipse  itself.  It  might  be  added  that 
von  Kliiber,  from  isophotes  drawn  from  microphotometer 
measures  of  the  1926  eclipse,  obtained  +  0.05  and  +0.02 
for  a  and  b  respectively,  in  excellent  agreement  with  Luden- 
dorff's  rougher  measures. 

Bergstrand *  also  takes  Ludendorff 's  figures  and  applies  a 
correction  to  the  values  of  the  isophotes  in  polar  regions  to 
allow  for  the  effect  of  equatorial  and  mid-latitude  streamers 
being  superposed  over  polar  rifts.  He  finds  a  correlation 
with  sun-spot  numbers  but  a  closer  connection  with  promi- 
nences in  high  latitude  zones. 

Lockyer z  makes  a  very  valuable  compilation  by  collecting 

1  Arkiv.  Mat.  Astr.  octi  Physik.  A,  22,  No.  i,  1930. 
-  Monthly  Notices,  R.  A.  S.  91,  797,  1931. 


CORONAL  THEORIES  403 

into  one  diagram,  beginning  with  the  year  1860,  information 
regarding  sun-spots,  prominences  and  coronal  shapes. 

On  account  of  the  connection  between  coronal  and  other 
solar  disturbances,  already  alluded  to,  and  on  account  of  the 
fact  that  prominences  are  much  more  widely  distributed  in 
heliographic  latitudes  than  are  spots,  it  is  not  surprising  that 
Lockyer  finds  a  close  connection  between  the  shape  of  the 
corona  and  prominences,  the  maximum  type  of  corona  oc- 
curring when  prominences  are  near  the  sun's  poles. 

The  quantities  a  and  b  in  the  table  are  exceedingly  inter- 
esting. The  mean  of  all  16  values  of  a  is  0.05.  The  only 
eclipse  with  outstanding  values,  that  of  1918,  is  explained  in 
Handbuch  der  Astrophysik. 

The  three  eclipses  1900,  1901  and  1922  took  place  on  the 
average  0.9  years  before  the  time  of  minimum  of  spots. 
Each  of  these  eclipses  showed  the  typical  "  minimum  "  co- 
rona; the  mean  value  of  b  is  0.26.  On  the  other  hand,  the 
three  eclipses  of  1914,  1923  and  1925  took  place  the  same 
interval  (0.9  years)  after  spot  minimum,  and  yet  the  coronas 
had  lost  their  minimum  characteristics,  b  having  the  mean 
value  0.15.  The  three  eclipses  1896,  1898  and  1930  took 
place  about  four  years  before  minimum  of  spots.  The  mean 
value  of  b  amounting  to  0.19  is  greater  in  size  than  that  at 
a  time  less  than  one  year  after  minimum.  Hence,  it  is  evi- 
dent that  two  years  before  spots  and  prominences  are  at  a 
minimum,  the  corona  takes  on  the  "  minimum  "  shape. 

With  the  recent  spot  maximum  occurring  at  1928.5,  the 
eclipses  of  1926,  1927  and  1929  were  all  "  maximum-type  " 
or  circular  coronas.  The  1926  eclipse  took  place  2^  years 
before  spot  maximum.  It  would  seem  also  that  the  "  maxi- 
mum "  type  of  corona  takes  place  two  years  before  maximum 
of  spots.  The  greatest  surprise  in  the  whole  of  the  tabular 
values  is  for  the  eclipse  of  October,  1930.  Taking  place  a 
year  and  a  half  after  the  1929  eclipse  and  only  a  little  more 
than  two  years  after  maximum  of  spots  and  probably  four 
years  before  the  next  minimum,  the  value  of  b  was  approxi- 
mately equal  to  that  of  a  minimum-type  of  eclipse. 

Bernheimer  l  agrees  with  the  writer  that  the  minimum 

1  Meddel.  Ltinds  Astron.  Obs.f  Ser.  I,  No.  126,  1931. 


404  ECLIPSES  OF  THE  SUN 

type  of  corona  takes  place  before  minimum  of  spots.  He 
also  finds  that  the  coronal  shapes  are  more  closely  connected 
with  sun-spot  numbers  than  with  the  phase  in  the  spot  cycle. 
After  observations  of  sun-spots  during  three  hundred  years 
we  are  forced  to  the  conclusion  that  the  n-year  curve  must 
be  regarded  as  quite  erratic.  The  solar  activity  from  num- 
bers and  areas  of  spots,  or  from  frequency  and  areas  of 
prominences,  gives  us  curves  which  are  similar  but  are  not 
identical.  Other  closely  allied  curves,  such  as  terrestrial  mag- 
netic phenomena,  at  times  depart  from  the  general  run  of 
the  spot  and  prominence  curves.  In  spite  of  numberless 
investigations,  it  is  still  unsafe  (see  p.  120)  to  predict,  even 
a  few  years  in  advance,  what  the  sun-spot  cycle  is  going  to 
do.  Recognizing  these  limitations,  or  in  other  words,  realiz- 
ing that  the  sun  has  sporadic  and  unexpected  bursts  of  ac- 
tivity, it  is  well  not  to  make  too  great  claims  for  the  de- 
pendence of  anything  on  the  sun-spot  curve. 

The  corona  takes  its  shape  primarily  from  the  lengths  and 
position  angles  of  the  longest  streamers.  The  eclipse  of 
1930  showed  that  the  longest  streamer  was  connected  with 
solar  activity  that  had  persisted  for  several  days.  Hence  it 
appears  certain  that  the  corona  can  have  no  constant  shape 
and  that  no  doubt  it  varies  from  day  to  day  depending  on 
whether  the  active  solar  areas  are  near  the  sun's  limb  when 
only  they  could  give  an  effect  projected  on  the  sky  back- 
ground. The  corona  we  happen  to  see  or  to  photograph 
during  the  few  fleeting  seconds  of  totality  is  a  temporary 
phenomenon  which  will  subject  itself  to  exact  analysis  with 
great  reluctance. 

In  spite  of  all  that  is  said  above,  the  author  is  going  to  be 
rash  enough  to  predict  that  the  1932  corona,  two  years  be- 
fore the  expected  minimum  of  spots,  will  show  the  minimum 
type  of  corona  with  long  equatorial  streamers  and  strong 
polar  brushes. 

When  we  attempt  to  solve  the  enigma  of  the  corona,  we 
are  face  to  face  with  one  of  the  most  difficult  problems  in 
the  whole  realm  of  astronomy. 

First  of  all,  the  corona  is  not  an  atmosphere  of  the  sun 
consisting  of  atoms  and  molecules  attracted  by  gravity.  But 


CORONAL  THEORIES  405 

gravity  on  the  sun  is  2  7  times  more  powerful  than  the  value 
on  the  earth,  while  the  corona  has  been  observed  to  the 
enormous  distance  of  ten  million  miles  from  the  surface  of 
the  sun.  Thus  it  is  easy  to  see  that  if  the  corona  were 
truly  atmospheric  in  its  nature,  the  resulting  pressure  would 
be  colossal.  The  chromosphere  is  indeed  the  solar  atmos- 
phere, not  one  of  oxygen  and  nitrogen  as  on  the  earth,  but 
of  the  gases  found  to  exist  in  the  flash  spectrum,  each  heated 
to  high  temperatures  due  to  their  proximity  to  the  sun,  and 
yet  the  pressure  at  the  base  of  the  chromosphere  is  much 
less  than  one-thousandth  of  the  earth's  pressure  at  sea- 
level. 

In  the  forceful  words  of  Simon  Newcomb1  we  may  re- 
mind the  reader  that  "  the  great  comet  of  1843  passed  with- 
in three  or  four  minutes  of  the  surface  of  the  sun,  and  there- 
fore directly  through  the  midst  of  the  corona.  At  the  time 
of  nearest  approach  its  velocity  was  350  miles  per  second, 
and  it  went  with  nearly  this  velocity  through  at  least  300,000 
miles  of  corona,  coming  out  without  having  suffered  any 
visible  damage  or  retardation.  To  form  an  idea  of  what 
would  have  become  of  it  had  it  encountered  the  rarest  con- 
ceivable atmosphere,  we  have  only  to  reflect  that  shooting 
stars  are  instantly  and  completely  vaporized  by  the  heat 
caused  by  their  encounter  with  our  atmosphere  at  heights 
of  from  50  to  100  miles;  that  is,  at  a  height  where  the  at- 
mosphere entirely  ceases  to  reflect  the  light  of  the  sun.  The 
velocity  of  shooting  stars  is  from  20  to  40  miles  per  second. 
Remembering,  now,  that  resistance  and  heat  increase  at 
least  as  the  square  of  the  velocity,  what  would  be  the  fate 
of  a  body,  or  a  collection  of  bodies  like  a  comet,  passing 
through  several  hundred  thousand  miles  of  the  rarest  at- 
mosphere at  a  rate  of  over  300  miles  a  second?  And  how 
rare  must  such  an  atmosphere  be,  when  the  comet  passes 
not  only  without  destruction,  but  without  losing  any  sensible 
velocity?  Certainly  so  rare  as  to  be  entirely  invisible,  and 
incapable  of  producing  any  physical  effect.''  Other  comets, 
the  great  comet  of  1882  for  instance,  have  almost  grazed 
the  sun's  surface. 

1  Popular  Astronomy,  265,  sixth  edition,  1887. 


406  ECLIPSES  OF  THE  SUN 

Any  adequate  theory  of  the  corona  must  be  capable  of 
explaining1  the  following  facts: 

1.  The  total  brightness  of  the  corona  is  very  small,  being 
about  one-half  that  of  full  moon  and  one-millionth  that  of 
the  sun.    Half  of  the  total  light  comes  from  a  zone  extending 
only  3'  from  the  limb  of  the  sun. 

2.  Its  spectrum  shows  the  bright  emission  lines  of  co- 
ronium  extending  to  a  maximum  distance  of  8',  and  also  a 
continuous  spectrum  in  the  inner  corona  and  Fraunhofer 
lines  in  the  middle  and  outer  corona. 

3.  The  emission  lines  of  the  spectrum  are  fainter  at  mini- 
mum of  spots  than  at  maximum. 

4.  The  emission  spectrum  due  to  "  coronium  "  contrib- 
utes only  a  small  fraction  of  the  total  energy  of  coronal 
light.     The  predominant  part  lies  in  the  continuous  spec- 
trum.   The  distribution  of  energy  in  this  continuous  spec- 
trum differs  little  from  that  of  the  sun. 

5.  Polarization  is  a  maximum  at  a  distance  of  about  5' 
from  the  sun's  limb,  and  it  diminishes  more  rapidly  towards 
the  sun  than  away  from  it. 

6.  Matter  of  any  kind  so  close  to  the  sun  must  be  very 
hot  and  must  reflect  and  scatter  the  solar  rays. 

7.  According  to  the  observations  of  Abbot,  the  coronal 
materials  are  deficient  in  heat  rays. 

8.  If  the  sizes  of  the  coronal  particles  change  in  diameter 
at  different  distances  from  the  sun's  limb,  a  corresponding 
change  in  color  of  the  corona  would  result.    No  change  in 
color  is  noticeable. 

9.  The  internal  motions  in  the  corona  are  very  small. 

10.  The  sun  exhibits  a  magnetic  field. 

11.  According  to  Bergstrand,  the  intensity  of  the  corona 
varies  inversely  as  the  square  of  the  distances  from  the  sun's 
surface.    Other  authorities  derive  the  inverse  fourth,  sixth, 
seventh  or  eighth  powers. 

12.  It  is  necessary  to  explain  the  changing  form  of  the 
corona  with  variation  in  the  sun-spot  period. 

In  the  bright  inner  corona  is  found  the  emission  spectrum 

1  See  also  Abbot,  Smithsonian  Misc.  Collections,  52,  31,  1908,  and  Lick  Ob- 
servatory Bulletin,  5,  15,  1908, 


CORONAL  THEORIES  407 

of  coronium  and  a  strong  continuous  spectrum.  In  the  less 
intense  outer  corona,  the  Fraunhofer  lines  are  seen.  In  the 
inner  corona,  due  to  proximity  to  the  sun's  surface,  are 
found  temperatures  approximating  those  in  the  photosphere. 
On  account  of  minute  pressures  existing  in  the  corona,  the 
atoms  may  readily  lose  one,  two,  or  more  external  electrons. 
The  long  free  paths  permitted  by  these  small  pressures 
cause  conditions  that  cannot  be  approximated  in  our  terres- 
trial laboratories.  Unfortunately,  due  to  the  paucity  of  light 
and  the  small  amount  of  time  available  for  investigations, 
coronal  wave-lengths  are  much  less  accurately  determined 
than  those  of  the  nebulae.  The  source  of  the  coronium 
lines  will  unquestionably  be  found  among  the  elements  in 
the  top  part  of  the  periodic  table.  The  "  coronium  "  prob- 
lem will  be  more  difficult  than  that  of  "  nebulium." 

As  the  coronium  emission  constitutes  a  very  small  part  of 
the  total  energy  found  in  the  corona,  it  may  practically  be 
left  out  of  consideration  in  attempting  to  find  a  theory  to 
explain  the  radiation  of  the  corona.  Any  adequate  theory 
must  give  solutions  to  the  following  problems:  (i)  What  is 
the  cause  of  the  strong  continuous  spectrum  of  the  inner 
corona,  and  ( 2 )  What  is  the  explanation  of  the  Fraunhofer 
spectrum  in  the  outer  corona? 

The  first  theory  of  the  corona  to  be  propounded  was  the 
one,  so  very  popular  at  the  time  and  thought  to  be  capable 
of  explaining  away  most  of  the  astronomical  problems  — 
the  meteoric  hypothesis.  In  virtue  of  this  theory,  the  corona 
is  nothing  more  nor  less  than  the  trails  of  myriads  of  meteors 
as  they  fall  into  the  sun.  Even  at  the  time1  a  great  au- 
thority on  meteors,  Newton  of  Yale,  pointed  out  that  the 
details  observed  in  the  corona  were  "  inconsistent  with  any 
conceivable  arrangement  of  meteoroids  in  the  vicinity  of  the 
sun."  The  hypothesis  was  such  an  artificial  one  and  had 
so  little  to  commend  itself  that  it  is  surprising  that  it  found 
such  a  large  place  in  astronomical  literature  —  but  then  we 
must  not  forget  that  very  little  was  known  concerning  the 
corona. 

Schaeberle's  mechanical  theory  of  the  corona  has  much 

1  Nature,  Sept.  30,  1866. 


408  ECLIPSES  OF  THE  SUN 

more  to  recommend  it  —  but  it  too  does  not  seem  to  have 
been  able  to  bear  the  test  of  time,  at  least  not  in  its  original 
form.  By  virtue  of  this  theory,  the  corona  is  caused  by 
light  emitted  and  reflected  from  streams  of  matter  ejected 
from  the  sun  by  forces  acting  along  lines  normal  to  the  sur- 
face of  the  sun.  The  forces  are  most  active  near  the  center 
of  the  sun-spot  zones,  and  consequently,  are  confined  almost 
wholly  to  the  equatorial  regions.  Hence,  as  a  result  of  this 
theory,  the  rays  seen  around  the  poles  of  the  sun  can  have  no 
existence,  except  that  of  streamers  from  the  equatorial  re- 
gions seen  projected  by  perspective  above  and  below  the 
poles.  In  order  that  the  force  of  ejection  may  be  suffi- 
ciently great  to  overcome  the  attraction  of  gravitation,  it 
was  necessary  to  ascribe  to  the  materials  forming  the  longest 
rays  initial  velocities  as  large  as  400  miles  per  second.  While 
velocities  of  this  size  are  not  impossible  on  the  sun,  the  truth 
of  the  matter  is  that  no  such  motions  have  ever  been  discov- 
ered in  the  corona.  Hence  it  is  necessary  to  discard  the 
theory  or  to  modify  it  in  some  essential  details.  Moreover, 
"  according  as  the  observer  is  above,  below,  or  in  the  plane 
of  the  sun's  equator,  the  perspective  overlapping  and  inter- 
lacing of  the  streamers  cause  the  apparent  variations  in  the 
type  of  the  corona."  This  explanation  might  satisfy  an 
annual  variation  (which  is  not  known  to  exist),  but  fails  to 
account  for  the  change  in  form  coincident  with  the  eleven- 
year  sun-spot  period. 

There  are,  unquestionably,  many  forces  acting  on  the  co- 
ronal materials  repelling  the  particles  away  from  the  sun 
in  opposition  to  gravitation,  and  as  these  forces  have  been 
considered  one  by  one,  different  coronal  theories  have  been 
propounded.  In  1885  the  "  electrical  theory  "  was  announced 
by  Huggins.1  The  eclipse  of  1889,  of  the  minimum  sun-spot 
type,  having  exhibited  strong  polar  rays  much  resembling 
the  lines  of  force  about  a  magnet,  Bigelow  L>  brought  forward 
the  "  magnetic  theory/'  and  Ebert 3  the  "  electro-magnetic 
theory."  Bigelow's  theory  is  very  successful  in  explain- 

1  Proc.  Royal  Society,  39,  108,  1885. 

2  The  Solar  Corona  discussed  by  Spherical  Harmonics,  1889. 

3  Astronomy  and  Astrophysics,  12,  804,  1893. 


CORONAL  THEORIES  409 

ing  the  details  of  the  minimum  type,  but  not  so  fortunate 
with  the  other  forms  of  corona.  The  recent  investigations 
at  Mt.  Wilson  have  shown  that  a  magnetic  field  does  exist 
around  the  sun,  as  demanded  by  Bigelow's  theory. 

Great  have  been  the  claims  of  the  exponents  of  the  "  ra- 
diation-pressure theory  "  not  only  for  explaining  the  details 
of  the  corona,  but  also  for  furnishing  a  rational  elucidation 
of  why  comets'  tails  always  point  away  from  the  sun,  and 
what  causes  the  aurora  borealis,  zodiacal  light,  etc.  That 
a  ray  of  light  exerts  a  pressure  on  any  surface  on  which  it 
impinges  comes  as  a  direct  result  of  the  Electro-magnetic 
Theory  of  Light  published  in  1873  by  Clerk  Maxwell1  and, 
as  was  shown  by  Bartolr  in  1876,  can  be  deduced  from 
the  second  law  of  thermodynamics. 

Since  the  light-pressure  depends  directly  on  the  intensity 
of  the  sun's  radiation,  which  decreases  inversely  as  the 
square  of  the  distance,  as  is  also  the  case  with  gravity,  the 
ratio  of  pressure  to  weight  is  therefore  a  constant  independ- 
ent of  the  distance  from  the  sun.  The  manner  in  which 
this  ratio  is  found  was  first  shown  by  Bessel 3  in  1836,  who 
computed  the  magnitude  of  the  repulsive  force  from  the 
curvature  of  the  tail  of  the  comet  in  1811.  Bredichin,4 
more  recently,  from  measures  of  many  comets'  tails,  has 
found  them  to  be  of  four  different  types,  in  which  the  re- 
pulsive forces  are  respectively  18.5,  3.2,  2.0,  and  1.5  times 
the  attraction  of  gravity;  the  straight  tail,  according  to  his 
ideas,  consisting  of  hydrogen,  the  plume-like  tail  of  hydro- 
carbons, and  the  short  stubby  one  of  metallic  vapors,  chief 
among  which  are  iron  and  sodium.  The  electrical  force,  on 
which  Bredichin  explains  his  repulsions,  has  been  shown  by 
Lebedew  5  not  to  have  a  sound  physical  basis. 

This  objection  cannot  be  raised  to  the  principle  of  Ar- 
rhenius.  That  light  actually  exerts  a  pressure  has  been 
shown  by  Lebedew,6  and  Nichols  and  Hull 7 ;  the  latter,  in- 

1  Electricity  and  Magnetism,  792. 
-  //  Nuovo  Cimento,  15,  195,  1883. 

3  A.  N.,  13,  185,  1836. 

4  Annales  de  VObservatoire  de  Moscou,  (2),  i,  45,  1886. 
r>  Astrophysical  Journal,  16,  155,  1902. 

6  Annalen  der  Physik,  (4),  6,  433,  1901;  Astrophysical  Journal,  15,  60,  1902. 

7  Physical  Review,  13,  307,  1901;  Astrophysical  Journal,  15,  62,  1902. 


410  ECLIPSES  OF  THE  SUN 

deed,  have  succeeded  l  in  producing  a  laboratory  comet's 
tail,  although,  as  pointed  out  by  them,  other  forces  than 
light-pressure  probably  helped  to  give  the  repulsion. 

However,  a  rigid  application  of  the  theory  of  Maxwell 
is  possible  only  when  the  body  acted  upon  is  large  compared 
with  the  vibrations  of  light  itself.  When  the  body  is  of  a 
size  approximating  the  wave-length  of  light,  Schwarzschild  " 
has  shown  that  the  maximum  value  of  the  repulsive  force  is 
about  twenty  times  the  attraction  of  gravity. 

Undoubtedly,  the  radiation-pressure  theory  has  been  of 
the  very  greatest  assistance  in  dealing  with  the  corona,  for 
the  reason  that  it  provides  us  with  a  knowledge  of  an  addi- 
tional force  acting  in  a  direction  in  opposition  to  gravitation. 
Miller  has  published  a  series  of  excellent  papers  3  enquiring 
into  the  question  whether  the  coronal  streamers  exist  in 
accordance  with  a  modified  Schaeberle  mechanical  theory, 
that  their  motions  are  produced  by  ejection,  by  the  rotation 
of  the  sun,  by  the  attraction  of  the  sun  and  by  the  radiant 
pressure  of  the  sun.  For  the  purpose  of  the  investigation, 
Miller  examined  the  excellent  series  of  photographs  of  the 
corona  obtained  by  the  Lick  Observatory  expeditions  from 
1893  to  1918,  inclusive,  also  plates  secured  by  himself  in 
1905  and  1918,  and  Lowell  photographs  taken  in  1918. 
The  conclusions  drawn  are  that  the  force  of  repulsion  is 
surprisingly  large,  being  almost  equal  to  the  attraction  of 
gravitation,  and  as  a  result,  it  is  unnecessary  to  assume  the 
very  large  velocities  of  Schaeberle's  original  theory.  The 
facts  accumulated  seem  to  be  in  fairly  satisfactory  accord 
with  the  theory  of  Arrhenius  expressed  as  follows:  *  "  It 
is  very  probable  that  those  drops,  for  which  gravitation 
is  just  compensated  by  the  pressure  of  radiation,  will  be  the 
chief  material  of  the  inner  corona.  For  drops  of  other  sizes 
are  selected  out,  the  heavier  ones  by  falling  back  to  the 
sun,  the  lighter  ones  by  being  drawn  away  by  the  pressure 

1  Astro  physical  Journal,  17,  352,  1903. 

2  Sitzungsberichte  der  math.-phys.     Classe  der  k.   b.  Akademie  der  Wissen- 
schaften  zu  Munchen,  31,  293,  1901. 

3  Astrophysical  Journal,  27,  286,  1908;  and  33,  303,  1911;  and  also  Publ.  A. 
S.  P.,  32,  207,  1920. 

4  Lick  Observatory  Bulletin 3  i,  152,  1902. 


CORONAL  THEORIES  411 

of  radiation,  so  that  just  those  drops  which,  so  to  say,  swim 
under  the  equal  influence  of  gravitation  and  pressure  of 
radiation  will  accumulate  in  the  corona." 

By  properly  choosing  his  mathematical  constants,  it  was 
possible  for  Miller  to  reproduce  each  and  every  coronal 
streamer  so  far  discovered  on  eclipse  photographs,  even  those 
streamers  found  near  the  sun's  poles.  Hence,  the  mechanical 
theory  supplemented  by  that  of  radiation  pressure  seems  to 
have  solved  many  of  the  perplexing  questions  regarding  the 
corona,  but  there  are  still  many  difficulties  to  be  surmounted 
before  we  can  feel  ourselves  on  thoroughly  sure  ground.  It  is 
unquestionably  necessary  to  take  into  consideration  mag- 
netic and  electric  forces.  Like  radiation  pressure  and  gravi- 
tation, these  forces  modify  the  other  effects  in  a  quantitative 
manner.1 

That  matter  can  exist  in  the  finely  divided  state  required 
by  the  theory  of  Arrhenius  was  shown 2  by  "  the  eruption 
of  Krakatoa,  which  drove  the  fine  ashes  up  to  an  eleva- 
tion of  30  km  ( 1 8  miles ) .  The  finest  particles  of  these  ashes 
were  slowly  carried  by  the  winds  to  all  parts  of  the  earth, 
where  they  caused,  during  the  following  two  years,  the  mag- 
nificent sunrises  and  sunsets  which  were  spoken  of  as  '  the 
red  glows/  This  glow  was  also  observed  in  Europe  after 
the  eruption  of  Mount  Pelee.  The  dust  of  Krakatoa  further 
supplied  the  material  for  the  so-called  £  luminous  clouds  of 
the  night/  which  were  seen  in  the  years  1883  to  1892  float- 
ing at  an  elevation  of  80  km  (50  miles),  and  hence  illumi- 
nated by  the  light  of  the  sun  long  after  sunset." 

If  the  temperature  of  the  inner  corona  is  approximately 
5000°  C,  it  is  quite  difficult  to  see,  as  was  pointed  out  by 
Abbot;"  how  matter  can  exist  in  the  solid  or  liquid  state 
or  how  the  dust-particles  of  the  inner  corona  can  be  "  drops 
of  liquid  metal."  4  Another  criticism  of  almost  insurmount- 
able character  is  that  voiced  by  Eddington.5  Owing  to  the 
fact  that  conditions  in  close  proximity  of  the  sun  cannot 

1  See  Pringsheim,  Physik  der  Sonne,  330,  1910. 

-  Worlds  in  the  Making,  7. 

:t  Lick  Observatory  Bulletin,  5,  20,  1908. 

4  Ibid.,  2,  1 88,  1904. 

5  Monthly  Notices,  R.  A.  S.,  80,  723,  1920. 


412  ECLIPSES  OF  THE  SUN 

be  duplicated  in  the  laboratory,  we  are  ignorant  of  the  true 
laws  of  radiation-pressure,  which  may  have  "  encouraged 
quite  exaggerated  ideas  of  the  possible  effects  of  radiation- 
pressure.''  The  "  upper  limit "  to  its  power  of  supporting 
or  driving  out  matter  has  been  calculated  by  Eddington, 
and  has  been  found  to  be  equivalent  to  a  pressure  of  2  dynes 
per  sq.  cm.  This  can  be  likened  to  a  wind  of  this  strength, 
and  the  exact  effect  of  any  material  will  depend  on  its  power 
of  absorption  —  of  stopping  the  wind  instead  of  letting  it 
blow  through.  Allowing  an  ample  margin  for  uncertainties 
of  observation,  Eddington  calculates  that  the  "  pressures 
of  radiation  cannot  carry  a  total  weight  of  more  than  a  milli- 
gram per  sq.  cm."  Applied  to  the  chromosphere,  the  den- 
sity is  found  to  be  of  the  approximate  size  of  io~12,  a  quantity 
which  indeed  appears  so  absurdly  small  that  it  seems  to 
contradict  all  our  ideas  concerning  prominences.  The  den- 
sity of  the  corona  and  in  comets'  tails  on  the  same  hypothesis 
is  a  thousand  times  smaller,  or  icr1 '. 

Apparently  therefore,  the  radiation-pressure  theory  is  not 
entirely  free  from  perplexities,  but  these  difficulties  may  not 
be  entirely  insurmountable.  Before  further  progress  is  made 
we  must  ascertain,  by  observational  means,  the  size  and 
the  number  of  particles  per  unit  volume  forming  the  corona 
at  different  distances  from  the  sun,  and  also  the  tempera- 
tures according  to  the  Stefan  and  Wien-Planck  formulas. 
The  luminosity  of  the  bright  inner  corona  is  caused  by  par- 
ticles which  are  heated  to  incandescence  by  solar  radiation 
and  which  scatter  sunlight,  the  particles  being  subject  to 
gravitation,  to  radiation  pressure  and  to  electromagnetic 
forces  largely  unknown.  Owing  to  their  proximity  to  the  sun, 
these  particles  cannot  be  in  the  solid  or  liquid  condition 
and  must  therefore  exist  in  the  gaseous  state.  The  mole- 
cules of  the  coronal  gases  strongly  illuminated  by  sunlight 
probably  act  like  the  fine  particles  of  a  fog  in  scattering  light. 
According  to  Fabry/  the  part  of  the  luminosity  of  the  corona 
which  gives  the  continuous  spectrum  may  be  due  to  this 
diffusion  and  not  to  reflection  by  small  particles.  As  the 
result  of  some  experiments  on  the  diffusion  of  light  by  air, 

1  Observatory,  41,  211,  1918. 


CORONAL  THEORIES  413 

Fabry  estimated  that  a  truly  gaseous  corona  having  a 
density  only  one-thousandth-millionth  that  of  air  at  at- 
mospheric pressure  would  scatter  sufficient  sunlight  to  ac- 
count for  the  luminosity  of  the  corona,  and  the  polarization 
effects  which  have  been  observed.  The  explanation  by 
Fabry  has  very  much  to  commend  it,  but  it  is  very  much 
to  be  doubted  whether  it  is  based  on  sufficient  experimen- 
tal evidence.  Moreover,  it  is  difficult  to  see  why  the  light 
of  the  corona,  if  caused  by  molecular  scattering,  is  white  in 
color  and  not  blue  like  the  sky. 

Since  the  heavenly  bodies,  the  sun,  the  stars,  nebulae, 
etc.,  are  bodies  at  high  temperatures  radiating  energy,  a 
knowledge  of  radiation  laws  is  of  vital  importance  to  the 
solution  of  astronomical  problems.  Unfortunately  until  re- 
cently, little  was  known  of  these  laws,  but  with  the  advent 
of  the  Bohr  atom  and  Saha's  theory  of  ionization,  but  spe- 
cially on  account  of  the  enormous  activity  in  the  past  decade 
in  investigating  problems  of  radiation,  we  have  already  made 
many  discoveries  and  many  more  are  to  follow. 

Saha's  theory  of  ionization  (see  Chapter  XVII)  has  al- 
ready been  very  successful  in  interpreting  the  importance  of 
enhanced  lines  in  the  flash  spectrum  and  in  furnishing  an 
adequate  explanation  of  the  differences  between  the  Fraun- 
hofer  spectrum,  the  sun-spot  spectrum  and  the  chromo- 
spheric  spectrum.  Cannot  this  same  theory  be  expanded 
so  as  to  furnish  an  adequate  explanation  of  the  cause  of  radia- 
tion in  the  corona? 

It  has  been  very  difficult  to  understand  the  process  where- 
by the  coronal  particles  are  enabled  to  emit  light  at  the  very 
great  distances  of  ten  million  miles  from  the  surface  of  the 
sun,  under  conditions  of  very  low  pressure  and  very  long 
mean  free  path.  But  the  corona  is  an  appendage  of  the  sun, 
and  somehow  or  other  (we  have  never  realized  just  how)  it 
may  be  possible  for  the  corona  to  borrow  some  of  its  radiation 
from  the  sun.  But  how  about  a  body  like  the  Orion  nebula? 
It  gives  a  bright-line  spectrum.  Are  astronomers  to  keep  on 
saying,  as  they  have  for  a  number  of  years,  that  the  Orion 
nebula  emits  light  because  the  kinetic  energy  causes  the 
particles  to  be  heated  and  that  the  luminosity  is  wholly  the 


414  ECLIPSES  OF  THE  SUN 

result  of  heat?  It  is  excessively  difficult  to  imagine  a  body 
of  such  vast  dimensions  heated  to  luminosity  but  yet  sur- 
rounded by  the  intense  cold  of  inter-stellar  space.  Ap- 
parently some  cause  other  than  that  of  temperature  must  be 
sought. 

In  studying  the  problem  of  galactic  nebulae,  Hubble1 
comes  to  the  conclusion  that  their  luminosity  is  derived  from 
the  radiation  from  stars  in  the  vicinity.  If  this  were  a 
simple  case  of  reflected  starlight,  the  spectra  of  the  nebula 
would  agree  exactly  with  the  spectra  of  the  associated  stars ; 
but  such  is  found  not  to  be  always  the  case.  Hubble  re- 
marks, "  It  is  doubtful  whether  or  not  a  mass  of  diffuse 
nebulosity  isolated  in  space  and  with  no  stars  involved  could 
hold  together  and  at  the  same  time  shine  by  light  generated 
by  collisions  of  molecules.  At  temperatures  corresponding 
to  intensity-distribution  or  width  of  lines  in  nebular  spectra, 
the  average  speeds  of  the  molecules  would  be  so  high  com- 
pared with  the  velocities  of  escape  that  the  nebulosities 
would  probably  dissipate  rapidly.  On  the  other  hand,  if 
molecular  speeds  were  sufficiently  small  to  admit  of  cohesion 
in  the  mass,  the  nebulosity  would  probably  be  too  cold  to 
radiate  light.  This  argument  suggests  that  diffuse  nebu- 
losity is  not  intrinsically  luminous,  but  is  rendered  so  by 
external  causes." 

If  it  is  possible  to  discover  the  mechanism  whereby  the 
nebulae  are  rendered  luminous,  the  same  mechanism  will 
probably  explain  the  cause  of  the  coronal  radiation.  The 
fact  must  not  be  overlooked  that  each  and  every  star  has 
a  corona  surrounding  it,  but  these  coronas  can  never  be 
rendered  visible  to  astronomers  since  the  total  light  of  each 
corona  must  be  comparatively  feeble  compared  with  the 
luminosity  of  the  sun  which  it  surrounds.  Milne  2  has  shown 
that  the  light  of  the  corona  can  have  no  sensible  effect  in 
modifying  the  spectrum  of  the  sun. 

The  mechanism  for  explaining  the  radiation  of  corona 
and  nebulae  unquestionably  is  found  in  the  electron.  The 
sun  and  the  stars  are  vast  radiating  spheres  at  very  high 

1  Mt.  Wilson  Contributions,  No.  241,  1922. 

2  Phil.  Trans.  Roy.  Soc'y,  A,  223,  201,  1922. 


CORONAL  THEORIES  415 

temperatures,  and  from  each  of  these  suns,  billions  of  elec- 
trons are  shot  off  every  second  of  time.  The  number  of  the 
electrons  emitted  depends  on  the  intensity  of  the  radiation 
while  the  energy  of  the  electrons  depends  primarily  on  the 
temperature.  The  energy  of  the  electrons  will  carry  them 
to  the  greatest  distances  from  the  radiating  body  in  the  case 
of  the  very  hottest  stars. 

Schwarzschild  proposes  a  theory  with  attractive  possi- 
bilities, namely,  that  the  corona  consists  of  "  electron-gas," 
that  is,  a  gas  of  very  long  mean  free  path  which  is  capable 
of  reflecting  and  polarizing  light.  But  as  Schwarzschild  has 
pointed  out,  the  electrons,  each  of  which  carries  a  unit 
negative  electric  charge,  must  require  an  equally  large  posi- 
tive charge  on  the  surface  of  the  sun;  this  causing  such  a 
strong  electrical  field  that  even  the  fastest  moving  electrons 
would  be  stopped  at  distances  less  than  a  millimeter.  In 
consequence,  the  corona  could  have  but  little  extension. 
Quite  independently,  Mitchell  (2d  ed.,  p.  356)  comes  to  the 
conclusion  that  the  radiation  of  the  outer  corona  is  caused 
by  electrons.  Ludendorff  attempts  to  get  rid  of  the  great 
electrostatic  charges  required  by  the  electron-gas  theory  by 
assuming  that  atoms  carrying  positive  charges  must  be 
mixed  with  the  free  electrons. 

In  a  series  of  excellent  articles,1  Wilhelm  Anderson  reviews 
the  various  coronal  theories.  He  accepts  the  electron- 
gas  theory.  After  making  certain  plausible  assumptions  con- 
cerning conditions  that  exist  in  the  corona,  he  calculates,  on 
the  basis  of  convective  equilibrium,  the  effective  molecular 
weight  of  the  coronal  material,  and  he  finds  a  good  agree- 
ment with  the  atomic  weight  of  the  electron.  The  thermal 
radiation  of  the  inner  corona  is  naturally  greatest  near  the 
solar  surface,  so,  likewise,  is  the  intensity  of  the  light  of  the 
photosphere  reflected  by  the  electrons.  Anderson  takes 
the  three  laws  of  intensities  of  radiation  in  the  corona,  the 
inverse  sixth,  fourth  and  square,  and  he  finds  a  good  agree- 
ment between  the  two  latter  at  a  distance  of  0.28  solar  radii. 
On  the  assumption  that  at  this  distance  the  diffuse  reflected 
light  has  38  per  cent  of  the  intensity  of  the  total  illumina- 

1  Zeits.  jur  Physik,  (4),  33,  1925. 


416  ECLIPSES  OF  THE  SUN 

tion,  he  calculates  the  results  from  theory  and  finds  a  good 
agreement  with  the  fourth-power  law  close  to  the  sun,  but 
at  greater  distances  out  a  better  agreement  with  the  inverse- 
square  law. 

It  therefore  seems  to  be  quite  probable  that  the  coronal 
radiation  takes  its  origin  in  the  electron,  the  energy  coming 
from  two  separate  causes:  (i)  from  thermal  radiation  re- 
sulting from  collisions,  and  (2)  by  reflection  and  scattering 
of  the  photospheric  light.  The  first  effect  is  visible  only  in 
the  inner  corona,  while  the  second  manifests  itself  both  in 
the  inner  and  the  outer  corona. 

In  quite  similar  manner,  Zanstra  1  has  made  an  applica- 
tion of  the  quantum  theory  to  explain  the  luminosity  of 
diffuse  nebulae.  The  nebula  appears  to  have  little  radiant 
energy  of  its  own,  but  borrows  or  takes  energy  from  the  star 
(or  stars),  the  electron  being  the  medium  for  the  transfer 
of  energy.  Bowen ~  explains  the  spectrum  of  nebulium  as 
the  result  of  the  long  mean  free  paths  of  the  electrons,  the 
jumps  being  "  forbidden  "  under  laboratory  conditions. 

Hence,  it  seems  to  be  definitely  proven  that  both  the  solar 
corona  and  the  nebulae  are  rendered  visible  as  the  result  of 
the  action  of  the  electrons.  Whatever  the  exact  mechanism 
may  be,  both  the  number  of  electrons  and  their  intensities  of 
emission  depend  on  the  temperatures  of  the  stars.  Accord- 
ing to  the  investigations  of  Eddington  and  Milne,  radiation 
pressure  greatly  assists  in  the  discharge  of  electrons. 

It  is  unnecessary  to  assume  with  Schwarzschild  that  the 
corona  consists  almost  exclusively  of  electron-gas.  Zanstra 
made  no  such  assumption  in  explaining  the  radiation  in  nebu- 
lae. In  the  inner  corona  is  found  the  emission  spectrum  due 
to  coronium.  This  radiation  can  be  caused  only  by  the  colli- 
sions of  electrons.  But  colliding  with  what?  Evidently 
with  protons  and  with  atoms  which  have  lost  one,  two,  or 
more  external  electrons.  Above  the  i4,ooo-km  level  reached 
by  the  highest  lines  of  the  chromosphere,  there  are  unques- 
tionably other  ions  reaching  far  greater  heights  but  whose 
radiations  give  no  light  in  the  visible  spectrum  but  are  found 

1  As  trophy  steal  Journal,  65,  50,  1927. 

2  Ibid.,  67,  i,  1928. 


CORONAL  THEORIES  417 

in  the  ultra-violet  beyond  the  reach  of  spectroscopic  investi- 
gations. Coronium  atoms  have  been  detected  to  distances 
twenty  times  greater  than  the  highest  in  the  chromosphere. 
Hence,  in  the  inner  corona  it  is  necessary  to  assume  ionized 
atoms  and  protons.  These  are  most  numerous  near  the  solar 
surface.  They  thin  out  rapidly  in  the  inner  corona  and  few 
of  them  are  found  in  the  outer  corona.  In  the  outer  corona 
there  can  be  relatively  few  ionized  atoms,  and  hence  the 
outer  corona  is  visible  mainly  as  the  result  of  scattering  of 
the  photospheric  light  by  the  electron. 

Regarding  the  shape  of  the  corona,  we  must  recall  that 
near  sun-spot  minimum  the  spots  in  the  dying  cycle  are  near 
the  solar  equator  while  those  awakening  into  life  are  at 
higher  latitudes.  Hence  when  the  sun  is  quiescent,  the  effect 
will  be  that  of  very  long  streamers  going  out  in  straight  lines, 
the  maximum  lengths  being  attained  not  at  the  equator  but 
at  the  higher  latitudes  of  the  awakening  spot  zone,  a  fact 
already  noticed  by  Campbell.  A  spot  on  the  sun,  or  an 
active  prominence,  may  be  a  local  center  of  activity  on  the 
sun,  with  the  result  that  coronal  streamers  or  hoods  sur- 
rounding the  prominences  may  result.  Since  the  field 
strength  in  spots  is  far  greater  than  that  of  the  sun  as  a  whole, 
the  comparative  inactivity  of  the  sun  at  its  poles  is  exhibited 
by  the  short  polar  brushes.  Even  before  passing  the  epoch 
of  minimum  of  spots,  the  sun  takes  on  new  energy,  and  hence 
electrons  are  discharged  with  increasing  strength  in  regions 
not  necessarily  limited  to  the  solar  equator.  Consequently, 
the  corona  takes  on  first  a  rectangular  shape,  and  then  a  con- 
tour more  and  more  circular  as  the  time  of  sun-spot  maxi- 
mum is  approached.  Owing  to  the  greater  average  vigor  of 
electronic  discharge  at  sun-spot  maximum,  there  should  be  a 
greater  intensity  of  the  emission  lines  of  coronium  than  at 
minimum  of  spots;  and  this  is  found  actually  to  be  the  case. 
After  passing  through  the  minimum  of  spots,  the  awakened 
solar  activity  shows  itself  in  three  different  portions  of  the 
sun:  (i)  In  the  photosphere,  the  increased  radiation  causes 
spots  to  appear.  (2)  In  the  chromosphere,  the  increased  ra- 
diation carries  the  elements  of  medium  height  to  greater  aver- 
age elevations.  (3)  In  the  corona,  the  increased  radiation 


418  ECLIPSES  OF  THE  SUN 

causes  an  increase  in  strength  of  the  emission  lines  of  coro- 
nium  and  also  makes  the  corona  lose  the  shape  associated 
with  minimum  of  spots,  of  long  equatorial  extensions  and 
short  polar  brushes. 

According  to  the  recent  work  of  many  investigators,  there 
is  some  doubt  existing  as  to  whether  the  atmospheres  of  stars 
are  in  thermodynamic  equilibrium.  If  they  are  not,  then 
the  result  will  be  that  the  theoretical  conclusions  now  ( 1932  ) 
believed  to  be  true  must  probably  be  modified  in  essential 
details.  Furthermore,  as  conditions  existing  in  the  sun  can- 
not be  approximated  in  laboratory  experiments,  little  is 
actually  known  of  the  exact  physical  laws  involving  the  dis- 
charge of  electrons  from  the  sun  or  those  underlying  radia- 
tion pressure  at  the  solar  surface.  It  seems  almost  useless 
to  test  any  theory  from  laboratory  experience. 

It  is  now  more  than  fifty  years  since  the  first  attempt  was 
made  by  Huggins  to  photograph  the  corona  in  full  sunshine. 
The  authority  of  his  name,  great  in  the  annals  of  spectros- 
copy,  gave  a  degree  of  plausibility  to  the  problem.  The 
task  to  be  overcome  is  to  separate  the  light  of  the  corona 
from  the  strong  illumination  of  the  sky.  The  chief  names 
connected  with  this  work  are  those  of  Hale,  Ricco,  Des- 
landres,  Wood  and  Hansky.  It  was  natural  that  the  meth- 
ods so  successful  in  photographing  the  prominences  should 
be  first  tried,  and,  in  order  that  the  atmospheric  glare  might 
be  reduced  as  much  as  possible,  the  observations  were  made 
from  mountain  tops,  Pike's  Peak  and  Mt.  Etna  being  oc- 
cupied for  this  purpose.  No  success  being  secured,  a  series 
of  attempts  were  made  by  heat-measuring  instruments  like 
bolometers  or  thermopiles.  Photographic  methods,  of  using 
color  filters  and  plates  sensitive  to  different  parts  of  the  spec- 
trum, have  been  thoroughly  tested.  Since  the  Great  War, 
when  such  noted  success  was  attained  in  airplane  photog- 
raphy by  using  plates  sensitive  to  the  deep  red,  attempts  were 
revived  on  the  corona.  Lindemann  had  found  it  possible  to 
photograph  stars  of  the  first  magnitude  near  the  sun.  Each 
and  every  one  of  the  plans,  however,  at  times  carried  out  with 
great  skill  and  ingenuity,  have  always  ended  in  the  same 
manner  —  failure  to  photograph  the  corona  in  full  daylight. 


CORONAL  THEORIES  419 

The  observations  by  Abbot  in  1908  and  those  by  Kunz  and 
Stebbins  in  1918  have  shown  the  cause  of  the  failures,  namely , 
the  intrinsic  feebleness  of  the  corona.  Even  in  the  brightest 
parts,  the  inner  corona  is  no  brighter  than  the  surface  of  the 
full  moon,  which  has  a  brilliancy  six-hundred-thousand  times 
less  than  the  sun.  The  corona  is  about  equal  to  the  intensity 
of  the  illuminated  sky  at  eight  or  ten  degrees  distant  from 
the  sun,  but  close  to  the  sun's  edge  the  light  of  our  central 
luminary  is  so  overpowering  that  it  appears  indeed  well- 
nigh  impossible  to  photograph  the  corona  in  full  daylight. 

In  view  of  the  above  conclusions,  it  is  interesting  to  read 
of  the  recent  work  1  of  Lyot  of  the  Meudon  Observatory.  At 
the  summit  of  the  Pic  du  Midi,  at  an  altitude  of  2800  meters, 
he  used  a  telescope  stopped  down,  and  with  a  disk  in  the 
focal  plane  extending  30"  beyond  the  sun's  edge.  Observ- 
ing directly  with  an  eyepiece,  he  saw  rose-colored  promi- 
nences. Setting  the  slit  of  a  spectrograph  close  to  the  limb, 
he  was  able  to  see  and  to  photograph  the  5303  and  6374 
coronal  lines,  superposed  on  the  Fraunhofer  spectrum.  Po- 
lariscopic  observations  were  made  and  direct  photographs 
were  obtained  which  resembled  in  appearance  the  inner  co- 
rona. It  is  surprising  to  find  success  where  others  had  failed. 
The  work  is  beset  with  such  great  observational  difficulties 
that  future  researches  will  be  followed  with  the  greatest  of 
interest. 

The  past  decades  have  been  golden  years  for  the  progress 
of  astronomy,  particularly  on  account  of  the  attack  on 
atomic  structure  by  the  astronomer,  physicist  and  chemist, 
combined.  The  importance  of  the  electron  has  thus  been 
recognized.  The  theory  of  ionization  which  has  already  been 
so  successful  in  furnishing  an  explanation  for  many  of  the 
difficulties  connected  with  the  flash  spectrum,  the  chromo- 
sphere and  sun-spots  is  but  a  branch  of  a  larger  theory  of 
photo-electricity  dealing  with  the  production  of  light  by  the 
passage  of  electricity  through  gases.  Photo-electric  action 
involves  both  ionization  and  radiation.  When  an  electron 
strikes  an  atom  and  a  transfer  of  energy  takes  place,  there 
may  be  complete  ionization,  as  shown  by  the  production  of 

1  Comptes  Rendus,  191,  834,  1930;  and  193,  1169,  1931. 


420  ECLIPSES  OF  THE  SUN 

positive  and  negative  ions,  or  there  may  be  partial  ionization, 
that  is,  a  disturbance  of  an  atom  which  is  not  detectable  as 
ionization  but  is  shown  by  the  production  of  radiation.  Both 
radiation  and  ionization  are  caused  by  the  action  of  electrons 
in  their  bombardment  of  atoms.  The  fundamental  basis  as- 
sumed for  Saha's  remarkable  theory  is  that  the  electrons  obey 
the  same  laws  as  gases,  or,  in  other  words,  that  they  have  the 
same  physical  properties  as  the  atoms. 

Moreover,  recent  work  in  astrophysics  has  demonstrated 
conclusively  that  the  source  of  energy  is  found  in  the  electron, 
that  even  mass  itself  has  its  origin  in  the  electron.  Edding- 
ton  has  shown  that  the  stars  are  all  slowly  losing  mass,  for 
the  reason  that  radiation  is  energy,  the  dissipation  of  energy 
means  the  discharge  of  electrons,  which  is  synonymous  with 
saying  a  diminution  of  mass.  Consequently  the  sun  and  stars 
are  continually  losing  mass  by  the  discharge  of  electrons. 
Hence  it  is  inevitable,  as  a  result  of  this  theory,  that  unless 
energy  is  being  created  —  possibly  by  cosmic  rays  —  the  end 
of  the  universe  may  be  foreseen. 

It  is  unfortunate  that  the  corona  can  be  investigated  only 
at  the  rare  occasions  of  total  eclipses  and  that  an  individual, 
no  matter  what  his  enthusiasm  or  skill,  will  have  during  his 
whole  lifetime  less  than  one  hour  within  which  to  make  all  of 
his  observations  of  the  corona.  In  the  immediate  future, 
total  eclipse  expeditions  mean  long  trips  from  home  to  loca- 
tions where  there  is  little  reason  to  expect  good  atmospheric 
conditions;  hence  progress  in  knowledge  concerning  the  co- 
rona promises  to  be  very  slow. 


CHAPTER  XXII 

THE    EINSTEIN    THEORY   OF    RELATIVITY 

EINSTEIN'S  theory  of  gravitation  has  been  justly  re- 
garded  as   the   greatest   triumph   of   mathematical 
reasoning  that  has  taken  place  since  the  time  of 
Newton.    It  is  safe  to  say  that  no  scientific  achievement  of 
recent  years  has  aroused  so  much  popular  interest  and  en- 
thusiasm as  that  evoked  by  the  verification  of  the  Einstein 
prediction  from  observations  made  at  the  total  eclipse  of 
May  29,  1919. 

The  first  step  in  any  scientific  investigation  is  to  get  at 
the  facts,  derived  from  observation  usually  by  a  series  of 
measurements.  Great  precision,  patience  and  care  are 
necessary  to  enable  the  observer  to  record  the  true  facts 
devoid  of  any  inferences  or  illusions  of  the  mind.  Lord 
Kelvin  has  said  that,  "  Nearly  all  of  the  greatest  discoveries 
of  science  have  been  but  the  rewards  of  accurate  measure- 
ment and  patient,  long-continued  labor  in  the  minute  sift- 
ing of  numerical  results."  After  the  observations'. have  been 
secured,  they  are  classified  and  analyzed  and  tested  to  see 
whether  they  will  conform  to  some  known  law.  The  laws 
of  nature  differ  greatly  from  human  or  civil  laws  since  the 
former  must  be  universally  true  and  must  apply  under  all 
conditions.  If  a  law  of  nature  is  found  to  be  deficient,  even 
in  some  minor  detail,  it  must  be  revised  to  satisfy  the  con- 
ditions, or,  if  this  is  impossible,  then  the  law  must  be  dis- 
carded. 

The  grandest  and  best  known  natural  law  is  that  of 
gravitation  discovered  by  the  great  Newton.  Its  importance 
lies  primarily  in  the  fact  that  it  applies  to  all  bodies,  it  is 
universal.  In  the  two  and  a  half  centuries  that  have  elapsed 
since  the  falling  of  the  apple,  the  law  of  gravitation  has 
experienced  one  grand  triumph  after  another,  with  the  in- 

421 


422  ECLIPSES  OF  THE  SUN 

evitable  result  that  we  have  come  to  look  upon  this  law  as 
practically  infallible.  Other  laws  may  come  and  go  and  be 
revised  into  changed  form,  and  go  on  again  to  be  again  and 
again  revised,  but  the  law  of  gravitation  has  stood  firm 
without  change,  with  the  enunciation  of  its  principle  in  the 
same  form  as  when  handed  to  us  by  Newton.  Every  body 
in  the  universe  attracts  every  other  body  in  the  universe 
with  a  force  that  is  proportional  to  the  product  of  the  masses 
and  inversely  to  the  square  of  the  distances  apart. 

And  now  we  are  told  that  the  law  of  gravitation  must 
be  discarded,  that  space  is  curved  and  that  we  live  in  a 
world  of  four  dimensions.  The  mind  reels,  and  refuses 
to  be  convinced.  Common  sense  tells  us,  we  say,  and 
the  experience  of  the  ages  has  proven,  that  space  has  but 
three  dimensions,  length,  breadth  and  thickness,  so  why 
say  such  an  apparently  foolish  thing  that  time,  which  we 
know  has  nothing  to  do  with  space,  must  be  considered  as 
a  fourth  dimension?  The  average  man  of  intelligence  im- 
mediately calls  to  mind  some  of  the  properties  of  the  mathe- 
matician's space  of  four  dimensions,  —  a  man  enclosed  in 
a  steel-proof  vault,  if  living  in  a  four-dimensional  space, 
could  get  outside  of  the  vault  without  passing  through  any 
of  the  walls.  One  smiles  incredulously,  and  thinks  of  a  re- 
mark by  Bertrand  Russell  that  "  mathematics  may  be  de- 
fined as  the  subject  in  which  we  never  know  what  we  are 
talking  about,  nor  whether  what  we  are  saying  is  true." 

Is  the  theory  of  relativity  of  Einstein  an  unreality  in  the 
brain  of  the  mathematician,  or  must  we  accept  it  as  true 
and  believe  in  a  four-dimensional  space?  If  the  Einstein 
theory  is  accepted,  then  as  an  important  consequence  the 
law  of  gravitation  must  be  revised,  there  is  no  middle  course. 
The  new  theory  states  that  there  is  no  "  force  "  of  gravita- 
tion, for  this  "  force  "  is  but  one  of  the  inherent  properties 
of  four-dimensional  space.  To  decide  which  of  the  two 
conflicting  hypotheses  best  represents  the  law  of  gravitation, 
Newton's  or  Einstein's,  we  shall  have  to  pass  in  review  the 
salient  facts.  Science  desires  the  simplest  explanation. 
Einstein's  own  conception  of  the  law  of  gravitation,  as  ex- 
pressed in  the  New  York  Times  is,  "  Please  imagine  the 


EINSTEIN  THEORY  OF  RELATIVITY  423 

earth  removed,  and  in  its  place  suspended  a  box  as  big  as 
a  room  or  a  whole  house,  and  inside  a  man  naturally  float- 
ing in  the  center,  there  being  no  force  whatever  pulling  him. 
Imagine  further,  the  box  being,  by  a  rope  or  other  con- 
trivance, suddenly  jerked  to  one  side,  which  is  scientifically 
termed  '  difform  motion,'  as  opposed  to  '  uniform  motion.' 
The  person  would  then  naturally  reach  the  bottom  on  the 
opposite  side.  The  result  would  consequently  be  the  same 
as  if  he  obeyed  Newton's  law  of  gravitation,  while,  in  fact, 
there  is  no  gravitation  exerted  whatever,  which  proves  that 
difform  motion  will  in  every  case  produce  the  same  effects 
as  gravitation. 

"  I  have  applied  this  new  idea  to  every  kind  of  difform 
motion  and  have  thus  developed  mathematical  formulas 
which  I  am  convinced  give  more  precise  results  than  those 
based  on  Newton's  theory.  Newton's  formulas,  however, 
are  such  close  approximations  that  it  was  difficult  to  find 
by  observation  any  obvious  disagreement  with  experience." 
Does  this  theory  of  Einstein  concerning  a  man  falling  from 
the  top  of  a  step-ladder  to  the  floor  of  a  room  really  furnish 
a  simpler  explanation  than  the  classical  law  of  falling  bodies 
of  Newton?  If  it  does,  then  a  very  radical  change  in  our 
modes  of  thought  must  take  place. 

History  shows  that  the  present  is  not  the  first  time  that 
the  perfect  science  of  astronomy  has  been  forced  to  change 
its  point  of  view.  In  the  early  days  of  civilization  it  was 
believed  that  the  earth  was  flat,  —  and  this  notion  is  still 
not  quite  eradicated  from  human  thought.  Later  was 
evolved  the  theory  that  the  earth  was  the  center  of  the 
universe  and  about  it  revolved  the  sun,  moon,  planets  and 
stars.  The  mechanism  required  by  the  Ptolemaic  system 
to  explain  the  motions  of  the  heavenly  bodies  by  cycles  and 
epicycles  was  such  a  cumbrous  one,  that  it  led  to  a  remark 
by  a  former  king  of  Spain  that  if  he  had  been  present  at 
Creation  he  could  have  furnished  the  Creator  with  some 
very  valuable  suggestions.  The  work  of  Copernicus,  Tycho, 
Kepler,  Galileo  and  Newton  revolutionized  scientific  thought, 
and  as  a  result  the  earth  was  displaced  from  her  important 
position  as  the  center  of  the  universe.  Although  the  geo- 


424  ECLIPSES  OF  THE  SUN 

centric  point  of  view  is  still  retained  in  some  of  our  scien- 
tific explanations,  we  realize  that  the  sun  is  the  center  of 
our  system,  with  the  earth  occupying  no  more  exalted  a 
position  than  that  of  being  but  one  of  the  planets.  The 
Newtonian  mechanism  reduced  celestial  motions  to  the 
greatest  simplicity.  In  obedience  to  the  law  of  gravitation, 
every  planet  describes  a  perfect  ellipse  about  the  sun  as 
focus,  and  these  elliptical  orbits  would  repeat  themselves 
indefinitely  were  it  not  for  the  gravitational  forces  arising 
from  the  other  planets.  After  allowing  for  all  of  the 
planetary  disturbances,  or  "  perturbations/'  Newcomb  found 
that  the  orbit  of  the  innermost  planet  Mercury  was  rotating 
in  its  own  plane  at  the  rate  of  42  seconds  of  arc  per  century 
in  excess  of  that  required  by  the  traditional  theory.  Vari- 
ous attempts  were  made  to  explain  away  this  discrepancy 
in  the  motion  of  Mercury  —  by  assuming  a  planet  or  planets 
inside  the  orbit  of  Mercury,  or  outside  of  its  orbit,  by  postu- 
lating a  belt  of  matter  in  a  flattened  disk  or  series  of  ellipses 
surrounding  the  sun,  or  by  presupposing  that  the  sun  has  an 
oblateness  of  shape  and  distribution  of  mass  different  from 
that  usually  taken  for  granted  —  but  all  to  no  avail.  In 
every  case,  the  assumption  of  mass  required  to  produce  the 
observed  effect  on  Mercury  would  have  caused  disturbances 
not  observed  in  the  other  planets.  The  solution  of  the 
problem  came  only  with  the  theory  of  relativity  which  fur- 
nished a  motion  of  the  perihelion  of  Mercury  of  43 "  per 
century,  in  almost  exact  agreement  with  the  observed  dis- 
crepancy. And  so,  when  in  addition,  the  observations  of  the 
1919  total  eclipse  gave  deflections  in  accordance  with  the 
Einstein  prediction,  the  confirmation  of  the  theory  of  rela- 
tivity seemed  almost  complete. 

Newton's  law,  however,  is  not  invalidated  by  the  latest 
discoveries  but  rather  is  supplemented  by  them.  It  is  pos- 
sible in  very  few  instances,  and  as  the  result  only  of  the 
most  refined  measurements,  to  be  able  to  detect  the  differ- 
ence in  observed  effect  between  the  Einstein  and  the  New- 
tonian laws.  In  addition  to  his  great  work  on  gravitation, 
Newton  proposed  an  erroneous  theory  of  light.  According 
to  this  theory,  light  consists  of  minute  corpuscles  expelled 


CANADIAN  PHOTOGRAPHS,  AUSTRALIA,  SEPTEMBER  21,  1922 
Upper  Pair:  Exposures  10  sec.  (left)   and  20  sec.  (right). 
Lower  pair:  Through  Nicol  prism  turned  90°  between  exposures,  each  of  5  sec. 


EINSTEIN  THEORY  OF  RELATIVITY  425 

from  a  luminous  source  with  high  velocities.  In  the  estima- 
tion of  Kayser,1  "  Newton's  errors  impeded  science  even  to 
the  middle  of  the  nineteenth  century."  It  was  not  until 
the  observations  of  Hooke,  Huyghens,  Young  and  Fresnel 
showed  the  corpuscular  theory  to  be  untenable  that  the 
wave  theory  was  finally  established.  But  how  do  the  waves 
of  light  travel  from  the  sun  to  the  earth?  It  was  necessary 
for  the  physicist  to  invent  some  material  medium  for  their 
propagation,  and  hence  the  existence  of  an  imponderable 
and  universal  "  ether  "  was  devised.  With  the  discovery 
of  the  laws  of  electromagnetism,  it  was  shown  by  Maxwell 
that  electromagnetic  disturbances  also  were  propagated  with 
the  speed  of  light,  and  the  conclusion  was  drawn  that  light 
itself  must  be  electromagnetic  in  character.  Additional 
functions  were  imposed  by  physicists  upon  the  universal 
medium  with  the  result  that  Maxwell  and  Kelvin  were  able 
to  make  estimates  of  the  density  and  rigidity  of  the  ether 
with  almost  as  much  assurance  as  if  they  had  been  dealing 
with  perceptible  matter.  (More  recent  researches  have 
shown  that  not  only  is  light  electrical  in  nature  but  so  also 
is  mass  and  energy.) 

Lis;ht  and  all  electromagnetic  manifestations  were  believed 
to  be  actions  taking  place  in  the  all-pervading  ether.  Light 
from  the  distant  stars  reaches  us  in  the  form  of  wave  mo- 
tions in  the  ether,  and  it  has  been  necessary  to  assume  that 
the  sea  of  ether  between  us  and  the  distant  stars  is  un- 
broken. Moreover,  in  order  to  account  for  astronomical 
aberration  and  other  phenomena,  it  was  proven  beyond  doubt 
that  the  ether  must  necessarily  be  at  rest,  that  it  pervades 
everything  but  that  it  is  not  dragged  about  by  ponderable 
material.  Since  motions  of  all  kinds  are  relative  in  their 
nature,  it  was  of  the  utmost  importance  to  theories  of  physics 
that  a  state  of  absolute  rest  should  exist  in  nature  and 
this  appeared  to  be  found  in  the  condition  of  the  ether. 
Accordingly,  attempts  were  made  to  detect  the  drift  of  the 
earth  through  the  motionless  ether.  In  1887,  the  first  ex- 
perimental determination  was  carried  out  by  Michelson. 
The  same  experiment  was  later  repeated  by  Michelson  and 

1  Handbuch  der  Spectroscopie,  i,  5,  1900. 


426  ECLIPSES  OF  THE  SUN 

Morley,  by  Morley  and  Miller,  and  quite  recently  by  Miller 
alone.  This  experiment  has  been  described  so  frequently 
that  it  will  be  unnecessary  here  to  go  into  details.  The 
velocity  of  light  was  known  to  be,  in  round  numbers,  186,000 
miles  per  second,  and  this  was  supposed  to  represent  the 
velocity  of  the  wave  motion  through  the  ether.  The  velocity 
of  the  earth  in  its  orbit  is  18  miles  per  second,  and  hence  if 
the  ether  is  stationary  with  respect  to  the  sun,  the  drift  of 
the  earth  through  the  ether  should  be  at  this  speed.  Ac- 
cordingly, if  a  beam  of  light  reaches  the  terrestrial  observer 
in  the  direction  of  the  earth's  motion,  it  will  be  traveling 
with  respect  to  and  overtaking  the  earth  at  a  speed  of 
185,982  miles  per  second,  while  if  the  beam  of  light  comes 
in  the  opposite  direction  its  velocity  with  respect  to  the 
observer  will  be  186,018  miles  per  second.  Although  it  is 
impossible  to  measure  the  velocity  of  light  along  a  single 
straight  course  with  sufficient  precision  to  compare  the  two 
velocities  above,  nevertheless  these  velocities  can  be  com- 
pared by  a  differential  method.  This  was  accomplished  by 
the  Michelson-Morley  experiment.  This  consisted  in  di- 
viding a  beam  of  light  by  means  of  an  unsilvered  mirror, 
allowing  each  half  of  the  beam  to  travel  the  same  distance 
in  directions  perpendicular  to  each  other,  and  then  reflecting 
the  two  beams  back  to  the  same  point  at  which  they  were 
originally  separated,  and  then  recombining  them.  It  is  evi- 
dent that  on  account  of  the  motion  of  the  earth,  the  path 
through  the  ether  of  the  half  beam  which  is  traveling  paral- 
lel to  the  direction  of  the  earth's  orbital  motion  must  be 
slightly  longer  than  the  other  half  of  the  beam.  Conse- 
quently, when  the  two  halves  of  the  beam  are  reunited,  the 
waves  of  light  should  be  a  little  out  of  phase.  By  turning 
the  apparatus  through  a  right  angle,  a  check  is  obtained  on 
the  equality  of  the  distances  traversed  by  the  two  beams. 
The  precision  of  the  experiment  was  so  great  that  it  should 
have  been  possible  to  detect  the  drift  of  the  earth  through 
the  ether  equal  to  one-tenth  of  its  orbital  velocity  about 
the  sun.  But  no  effect  whatever  was  detected  and  there 
was  no  indication  that  the  earth  was  moving  at  all  through 
the  ether. 


EINSTEIN  THEORY  OF  RELATIVITY  427 

The  most  obvious  explanation  of  the  failure  of  the 
Michelson-Morley  experiment  to  detect  the  drift  of  the 
earth  is  that  the  ether  is  stationary  with  respect  to  the  earth, 
or  is  carried  along  by  it.  But  if  we  admit  this  possibility, 
we  are  in  a  quandary  to  explain  the  aberration  of  light  from 
the  stars.  Everyone  is  familiar  with  the  fact  that  on  a  rainy 
day  when  there  is  no  wind  the  rain  falls  vertically,  and  to 
keep  dry,  when  standing  still,  it  is  necessary  to  hold  an  um- 
brella over  one's  head.  If  the  holder  of  the  umbrella  starts  to 
walk  he  tilts  the  umbrella  forward  in  the  direction  of  his  mo- 
tion, for  experience  has  told  him  that  only  by  so  doing  can  he 
keep  dry.  The  faster  he  walks  the  more  the»umbrella  is  tilted 
forward.  This  is  a  simple  case  of  relative  motion.  So  when 
a  telescope  is  pointed  in  the  direction  of  a  star,  the  telescope 
tube  must  be  tilted  forward  by  an  appreciable  angle  so  that 
the  rays  from  the  star  may  pass  down  the  tube  to  the  eye 
of  the  observer.  The  angle  through  which  the  telescope  has 
to  be  inclined  is  known  as  the  angle  of  aberration.  This 
angle  was  discovered  by  Bradley  in  1728.  Its  value  is 
2o"47.  On  account  of  the  great  precision  of  modern  astro- 
nomical measurements  this  angle  is  considered  large. 

But  how  explain  the  Michelson-Morley  paradox  that  the 
earth  must  be  drifting  through  the  ether  while  at  the  same 
time  the  effect  of  this  drift  has  never  been  detected?  Ap- 
parently there  is  no  way  out  of  the  dilemma  but  that  our 
ideas  must  in  some  manner  be  revised.  In  1893,  a  satis- 
factory explanation  was  offered  by  Fitzgerald,  and  in  1895 
independently  by  Lorentz,  that  the  negative  result  of  this 
experiment  becomes  quite  intelligible  if  it  is  assumed  that 
when  a  body  is  in  motion  through  the  ether  its  dimensions 
in  the  direction  of  motion  become  slightly  shorter  than  when 
it  is  at  rest.  This  would  indeed  seem  to  be  a  strange  and 
a  highly  arbitrary  hypothesis  if  unsupported  by  other  scien- 
tific evidence.  The  work  of  Lorentz  and  Larmor  on  the 
electromagnetic  theory  showed  that  it  was  necessary  to  as- 
sume that  a  body  when  in  motion  does  actually  contract 
by  just  the  amount  demanded  by  Fitzgerald's  explanation. 
If  this  hypothesis  is  true,  then  as  a  result  we  are  inevitably 
forced  to  the  conclusion  that  time  also  must  be  measured  in 


428  ECLIPSES  OF  THE  SUN 

a  different  manner  for  an  observer  at  rest  and  for  one  in 
motion.  The  Fitzgerald  contraction  is  not  merely  an 
idle  arbitrary  speculation,  but  after  repeated  experiments 
it  has  been  shown  to  be  true  and  mathematically  exact  in 
the  well-known  laws  of  electromagnetic  forces. 

The  amount  of  contraction  depends  on  the  velocities, 
and  in  the  majority  of  the  cases  of  our  experience,  or 
for  average  velocities,  is  excessively  minute.  In  the  case 
of  the  earth  the  contraction  is  only  one  part  in  200,000,000, 
which  corresponds  to  a  contraction  of  2^  inches  in  the 
earth's  diameter  in  the  direction*  of  its  motion.  The 
Michelson-Morley  experiment  therefore  failed  to  detect 
the  motion  through  the  ether  for  the  simple  reason  that 
the  dimensions  of  the  apparatus  were  actually  automatically 
contracted  by  an  amount  just  sufficient  to  compensate  for 
the  effect  sought.  Other  ingenious  experiments,  electrical 
and  optical,  were  tried  for  the  purpose  of  detecting  the  drift 
through  the  ether,  but  always  with  the  same  result.  There 
thus  appears  to  be  a  "  conspiracy  "  among  the  various 
agencies  at  work  to  prevent  man  from  measuring  his  motion 
through  space. 

In  1905,  Einstein  published  his  restricted  Principle  of 
Relativity  that  "  it  is  of  necessity  impossible  to  determine 
absolute  motion  by  any  experiment  whatever."  According 
to  Einstein,  all  motion  is  relative,  and  no  experiment  can 
be  possibly  devised  so  as  to  decide  which  of  two  systems 
is  at  rest  and  which  in  motion.  It  is  therefore  impossible 
by  any  experiment  to  detect  uniform  motion  with  respect 
to  the  ether.  Hence  the  assumption  of  the  ether,  brought 
into  physical  science  for  no  other  purpose  than  to  explain 
the  propagation  of  light,  becomes  entirely  unnecessary.  If 
the  ether  has  no  position  whatever  in  space,  the  statement 
that  it  exists  has  no  meaning.  Hence,  if  the  ether  is  retained 
by  the  physicist  he  must  assign  a  new  set  of  properties. 
A  second  consequence  is  that  the  velocity  of  light  with 
respect  to  two  observers  moving  relatively  to  each  other 
must  always  be  the  same,  no  matter  in  what  direction  the 
light  is  traveling,  as  the  velocity  must  be  equivalent  to 
that  when  determined  by  an  observer  at  rest,  of  186,000 


EINSTEIN  THEORY  OF  RELATIVITY  429 

miles  per  second.  These  radical  changes  can  exist  only 
under  the  condition  that  certain  relations,  known  as  rela- 
tivity transformations,  exist  between  the  space  and  time 
measurements  made  by  the  two  observers. 

The  principle  of  relativity  thus  entails  changes  in  our 
mode  of  thought  of  the  most  revolutionary  kind,  and  our 
preconceived  notions  in  consequence  are  turned  topsy-turvy. 
Let  us  see  a  few  examples.  If  an  observer  is  traveling  at 
the  rate  of  161,000  miles  a  second  in  a  vertical  direction  his 
arm,  30  inches  long,  is  of  its  natural  length  when  extended 
horizontally,  but  is  contracted  to  15  inches  when  hanging 
by  his  side  or  raised  vertically  above  his  head.  This  you  say 
is  foolishness.  Well,  take  a  yardstick  and  measure  the 
length  of  the  arm.  Horizontally  the  arm  measures  30  inches 
on  the  scale,  and  vertically  exactly  the  same  30  inches  by  the 
scale.  Yes,  but  when  you  hold  the  scale  vertically  it  too 
has  been  contracted  and  each  inch  on  the  scale  is  in  reality 
only  half  an  inch,  so  that  the  arm  measures  30  half -inches  or 
15  inches.  "Yes,"  you  will  say,  "but  whoever  heard  of 
anyone  moving  with  such  a  colossal  speed  as  161,000  miles 
per  second?  "  Why  not?  It  is  impossible  to  measure  ab- 
solute velocities  in  space,  and  for  all  we  know  to  the  con- 
trary we  may  be  speeding  along  at  this  terrific  rate.  The  ft 
particles,  referred  to  in  Chapter  XIV,  move  with  velocities 
of  100,000  miles  per  second. 

A  conclusion  perhaps  even  stranger  than  the  foregoing 
must  be  drawn  from  the  principle  of  relativity.  Suppose 
observer  A  is  moving  through  space  at  a  speed  of  161,000 
miles  per  second  relative  to  B,  and  allow  each  observer  to 
hold  an  identical  yardstick.  On  account  of  his  fast  flight,  A's 
stick  is  contracted  to  half  of  the  length  it  has  when  at  rest, 
and  hence  B's  stick  appears  to  be  twice  the  length  of  his 
own.  But  since  motion  is  purely  relative  in  character,  B 
must  be  moving  with  respect  to  A  with  the  speed  of  161,000 
miles  per  second,  and  hence  B's  yardstick  is  contracted;  and 
it  appears  to  be  twice  as  long,  half  as  long,  or  equal  to  the 
length  of  an  identical  stick,  depending  on  which  stick  is  the 
moving  one  or  whether  they  both  are  at  rest. 

If  the  speed  of  the  observer  is  increased  beyond  161,000 


430  ECLIPSES  OF  THE  SUN 

miles  per  second ,  the  contraction  becomes  greater  and  still 
greater;  and  at  the  velocity  of  light  all  lengths  dwindle  to 
zero  and  vanish.  The  observer  changes  to  an  object  of  two 
dimensions,  length  and  breadth,  but  without  thickness,  but 
he  himself  is  blissfully  unconscious  of  his  sorry  plight.  Dis- 
tance is  annihilated  and  so  also  is  time.  If  traveling  at  the 
velocity  of  light,  the  observer  could  go  to  the  most  distant 
star  without  becoming  a  day  or  even  a  second  older,  for  the 
reason  that  no  time  had  elapsed.  These  paradoxes  make 
the  theory  of  relativity  very  difficult  of  comprehension  to 
anyone  who  is  not  a  mathematician  and  they  seem  moreover 
to  controvert  our  "  common-sense  "  view  of  things. 

Everyone  will  agree  that  if  one  measuring  rod  identical 
in  length  with  another  can  be  twice  as  long,  half  as  long  or 
equal  in  length  to  the  other,  there  must  be  something  wrong 
somewhere  with  the  method  of  measurement.  Einstein  in- 
quired into  our  methods  of  measuring  length  and  time  and 
pointed  out  the  lack  of  definiteness  in  the  concepts  of  space 
and  time  as  ordinarily  used.  What  do  we  do  when  we  meas- 
ure the  length  of  an  object  like  a  book?  We  take  a  foot-rule 
or  meter  stick  and  determine  its  "  true "  length.  We 
measure  the  size  of  the  book,  however,  only  by  comparing  it 
with  a  scale  whose  length  we  suppose  to  be  known.  But 
what  do  we  actually  know  of  the  length  of  the  foot-rule? 
We  take  it  for  granted  that  it  has  been  compared  with  some 
standard  foot-rule  or  yardstick.  We  know  the  lengths  of 
our  measuring  rods  only  by  comparing  their  lengths  with 
other  rods,  and  so  on  and  on  through  an  almost  endless  series 
of  comparisons,  until  we  come  to  the  Standard  Yard  pre- 
served in  London,  or  the  Standard  Meter  kept  in  Paris. 
The  latter  is  a  rod  of  definite  shape,  of  a  certain  particular 
metallic  composition,  and  at  a  given  temperature,  the  length 
between  two  specified  lines  is  exactly  a  meter.  The  length 
of  the  meter  is  approximately  one  ten-millionth  part  of  the 
earth's  quadrant.  What  would  happen  if  the  earth  and  all 
it  contains  were  suddenly  contracted  to  half  its  present 
dimensions?  All  objects  including  ourselves  and  all  measur- 
ing appliances  would  dwindle  to  half  size.  We  would  never 
have  any  idea  that  the  earth  had  contracted  if  we  kept  our 


EINSTEIN  THEORY  OF  RELATIVITY  431 

attention  on  terrestrial  objects.  But  lo,  and  behold,  the 
sun  has  suddenly  doubled  its  size!  If  the  sun  and  the  ex- 
ternal universe  could  have  contracted  at  the  same  time  as 
the  earth  and  the  observer,  we  could  never  have  become 
aware  of  the  difference.  The  "  true  "  length  of  a  book  or 
a  rod  thus  depends  on  the  definitions  and  postulates  which 
we  adopt.  Manifestly  there  is  no  such  thing  as  "  absolute  " 
space,  everything  is  merely  relative  to  the  observer. 

You  will  insist,  however,  that  if  there  is  no  "  absolute  " 
space,  there  must  certainly  be  "  absolute  "  time,  for  time 
seems  to  go  on  quite  independently  of  any  observer.  Each 
of  us  unconsciously  feels  that  when  we  die,  the  world  will 
carry  on  just  about  as  before.  Time  and  space  thus  seem 
on  a  very  different  basis.  But  how  do  we  measure  time? 
Ordinarily  by  a  clock.  We  assume  that  the  pendulum  swings 
in  a  perfectly  uniform  manner  and  that  the  clock  hands  move 
equal  distances  in  equal  intervals  of  time.  But  how  prove 
our  assumption?  We  are  forced  for  our  definitions  of  time 
intervals  to  go  back  to  the  earth,  and  to  assume  that  the 
earth  is  a  colossal  top  set  spinning  on  its  axis,  and  its  mass 
is  so  great  that  like  a  gigantic  fly-wheel  its  motion  must  cer- 
tainly be  uniform.  The  assumption  seems  sound  and  all 
of  the  accurate  measurements  of  astronomy  are  based  on  it. 
What  would  happen  to  the  vaunted  predictions  of  as- 
tronomy if  the  earth  changed  its  speed  of  rotation?  In  fact, 
it  was  shown  in  Chapter  IV  that  some  suspicion  has  lately 
been  cast  on  the  uniformity  of  rotation  of  the  earth,  —  and 
our  time-keeper  may  after  all  be  a  faulty  one.  Apparently 
therefore,  we  seem  forced  to  the  conclusion,  since  time- 
measurements  depend  on  measures  of  distances,  that  there 
is  no  absolute  time  any  more  than  there  is  absolute  space. 
The  determination  of  both  space  and  time  consist  essentially 
in  the  measurement  of  certain  lengths,  and  such  measures 
have  no  meaning  except  in  relation  to  ourselves  and  our 
every-day  experiences. 

Let  us  return  again  to  the  measurements  of  the  book.  We 
recognize  the  object  because  we  have  seen  other  books.  We 
glance  inside  the  pages,  and  find  a  treatise  on  higher  mathe- 
matics or  some  ebullition  of  modern  fiction,  both  unreal  and 


432  ECLIPSES  OF  THE  SUN 

devoid  of  meaning  to  our  feeble  intellect.  We  likewise 
recognize  the  foot-rule  and  we  accept  some  one's  word  for 
it  that  this  rule  has  been  compared  with  other  rods,  etc.,  etc., 
and  that  we  know  its  length.  Applying  the  measuring  rule 
to  the  book,  and  spending  a  few  seconds  of  time  in  the  opera- 
tion, we  find  a  length  of  eight  inches.  A  Martian  dropped 
to  the  earth  would  not  recognize  a  book,  for  in  the  advanced 
civilization  that  is  supposed  to  exist  on  the  ruddy  planet, 
books  do  not  exist.  Neither  would  he  recognize  the  foot- 
rule,  and  certainly  the  marks  that  designate  the  inches  would 
be  utterly  unintelligible.  The  simple  process  of  measuring 
the  length  of  a  book  would  be  completely  impossible  to  the 
gigantic  brain  of  the  Martian.  He  has  not  been  taught 
from  his  childhood  up  to  measure  with  our  particular  rods. 

The  measurement  of  a  length,  simple  though  it  seems,  is 
after  all  a  very  complicated  process.  We  see  the  book,  and 
note  that  it  occupies  a  definite  position  on  a  table  in  a  room 
furnished  as  a  library.  Having  two  eyes,  we  have  learned 
unconsciously  to  estimate  the  distance  to  the  book,  and  so 
we  telegraph  the  thought  to  our  brain  that  we  must  reach 
forth  the  hand  and  pick  up  the  book.  (It  is  not  necessary 
to  trace  further  the  train  of  thought.)  But  it  is  easy  to  see 
that  in  measuring  the  length  of  the  book  we  are  putting  into 
practice  the  training  of  hundreds  and  even  thousands  of 
years,  acquired  at  times  with  great  difficulty  by  ourselves 
and  our  ancestors. 

Knowledge  thus  comes  to  us  only  as  a  series  of  experiences 
or  events.  Each  experience  is  perhaps  an  instantaneous 
photograph  on  the  brain.  But  every  photograph  when  taken 
requires  a  definite  exposure-time,  sometimes  longer,  some- 
times shorter  in  length,  depending  on  the  intensity  of  the 
light  and  the  speed  of  the  photographic  plate.  And  so 
there  are  brains  which  work  slowly  and  those  which  grasp 
more  quickly.  We  see  therefore  that  throughout  all  of  our 
existence  we  have  tacitly  assumed,  without  realizing  it,  that 
measures  of  space  are  not  at  all  independent  of  time,  and 
that  durations  of  time  are  not  devoid  of  their  inherent  refer- 
ence to  space.  There  are  no  durationless  events,  and  each 
and  every  one  of  our  experiences  involves  a  consciousness 


EINSTEIN  THEORY  OF  RELATIVITY  433 

of  both  space  and  time.  Thus  neither  space  nor  time  is 
absolute,  and  every  observation  is  relative  to  the  individual 
observer.  But  each  observer  being  confined  to  the  earth, 
by  common  consent  we  have  come  to  the  conclusion  that 
Truth  is  attained  only  when  our  observations  agree  with 
that  of  the  average  individual  and  that  we  are  in  error  when 
we  differ  from  the  general  average. 

Throughout  the  whole  of  our  scientific  consciousness  we 
have  assumed  that  in  agreeing  with  the  average  observer 
we  have  thereby  derived  the  "  true  "  length  or  have  de- 
termined the  "  true  "  duration  of  time.  The  great  triumph 
of  Einstein  consisted  in  making  clear  that  the  "  true  " 
measures  of  each  observer,  and  hence  of  the  average  ob- 
server, are  only  relative  in  their  nature  and  not  absolute  in 
value.  Consequently,  all  of  the  observations  with  which 
we  are  familiar  are  relative  and  depend  only  on  the  indi- 
vidual as  represented  by  the  average  observer.  The  method 
of  advance  was  manifestly  to  divorce  the  series  of  events 
from  the  observer,  and  look  upon  them  from  the  point  of 
view  of  the  objects  themselves.  This  was  Einstein's  method. 
He  assigned  space  and  time  solely  to  the  observer,  but  gave 
to  nature  an  unfamiliar  combination  of  space  and  time  of 
four  dimensions.  However,  this  is  not  the  same  four- 
dimensional  space  that  has  lately  been  much  discussed  by 
the  mathematician,  a  space  of  length,  breadth  and  thickness 
and  a  fourth  dimension  at  right  angles  to  the  other  three. 
The  relativity  world  of  four  dimensions  we  have  in  reality 
been  quite  familiar  with  throughout  our  whole  lives,  as  is 
signified  by  the  words  right-and-left,  forward-and-backward, 
up-and-down,  and  sooner-and-later.  The  first  three  dimen- 
sions give  the  familiar  world  of  objects,  the  four  dimensions 
taken  together  furnish  the  world  of  events.  In  this  four- 
dimensional  world  a  particle  occupies  not  one  point,  as  we 
are  accustomed  to  think  of  in  space  of  three-dimensions,  but 
a  series  of  points  representing  its  positions  at  successive  in- 
stants of  time.  Its  history  then  is  represented  by  a  line, 
called  its  "  world-line."  If  an  observer  is  admitted,  he  has 
his  own  world-line  and  immediately  he  imagines  that  his 
world-line,  of  all  lines,  is  the  most  important  in  the  universe. 


434  ECLIPSES  OF  THE  SUN 

He  immediately  proceeds  to  divide  up  the  four-dimensional 
entity  into  a  space  of  three  dimensions  with  time  as  the 
fourth,  and  we  have  the  world  familiar  to  us  all.  But  the 
theory  of  relativity  rules  out  the  observer  from  any  con- 
sideration whatever,  and  there  now  results  a  space-time 
nature  in  which  is  scarcely  recognized  space  and  time. 
There  is  thus  no  "  shape  "  to  anything,  no  difference  be- 
tween straight  and  crooked,  it  is  impossible  to  measure  an 
angle,  which  is  in  fact  the  difference  in  direction  between 
two  straight  lines.  This  four-dimensional  world  which  was 
invented  by  Minkowski,  naturally  has  no  reality,  it  does  not 
exist  and  its  properties  cannot  be  interpreted  by  our  ordi- 
nary experiences.  The  fourth  dimension  at  right  angles  to 
the  other  three  is  not  even  the  time  plotted  as  a  quantity 
directly,  but  the  product  of  time  into  the  square  root  of 
minus  one.  This  quantity,  V  —  ~i,  is  known  to  every  in- 
telligent boy  as  the  symbol  of  an  imaginary  quantity.  Hence 
it  is  futile  to  attempt  to  assign  to  this  unfamiliar  and  unreal 
world  any  of  the  attributes  derived  from  the  common  ex- 
perience of  life.  Many  writers  in  their  attempts  to  popular- 
ize have  used  such  catch-phrases  as  "  time  is  curved,"  "  there 
is  no  now"  "  a  phenomenon  may  be  seen  before  it  happens/' 
and  so  forth  and  so  on. 

"  Relegating  space  and  time  to  their  proper  source  —  the 
observer  —  Einstein  bids  us  contemplate  the  residuum  of 
what  we  observe.  This  residuum  is  the  true  world.  It  is 
shapeless,  because  we  have  abstracted  shape;  yet  it  is  metri- 
cal and  has  quantitative  properties  which  can  be  expressed 
in  mathematical  terms.  Clearly  we  cannot  describe  this 
true  world  in  terms  of  familiar  things,  because  the  whole 
point  of  Einstein's  theory  is  that  we  must  subtract  the  ideas 
which  we  ourselves  have  added  in  order  to  form  familiar 
things.  Mathematics  is  the  only  language  in  which  the  in- 
herent qualities  of  this  unfamiliar  world  can  be  described. 
But  though  it  seems  unfamiliar,  nature  is  left  simpler  by  this 
purification.  A  closer  unity  is  perceived  in  the  bases  of 
phenomena  apparently  diverse;  and,  for  example,  the  effect 
of  gravitation  on  light  is  clearly  foreseen.  Further,  the  laws 
of  nature  must  relate  to  this  four-dimensional  residuum  and 


EINSTEIN  THEORY  OF  RELATIVITY  435 

the  space  and  time  we  ourselves  introduce  cannot  be  rele- 
vant. This  led  Einstein  to  the  conclusion  that  Newton's 
law  of  gravitation,  which  refers  to  one  particular  separation 
of  space  and  time,  cannot  be  the  exact  law;  and  he  proposed 
a  new  law  applicable  to  the  four-dimensional  world.'3 1 

Although  there  is  no  distinction  between  straight  and 
crooked  in  the  four-dimensional  entity,  it  is  however  possible 
to  join  two  points  or  events  by  a  certain  unique  track  which 
plays  a  part  corresponding  to  the  straight  line  in  our  familiar 
life.  This  unique  track  is  called  a  geodesic,  and  instead  of 
being  the  shortest  distance  between  two  points  —  the  im- 
portant property  of  the  straight  line  in  ordinary  space  — 
the  geodesic  is  of  the  maximum  length.  We  see,  therefore, 
what  unfamiliar  conceptions  are  involved  in  the  theory  of 
relativity,  and  how  radically  changed  must  be  the  point  of 
view  that  makes  the  distance  between  two  points  a  maximum 
and  not  a  minimum ! 

Let  us  see  some  of  the  necessary  consequences  of  the 
principle  of  relativity  in  its  relation  to  the  propagation  of 
light.  As  already  stated,  this  principle  enunciates  that  the 
velocity  of  light  is  independent  of  the  observer,  or,  stated 
in  other  words,  that  no  matter  what  is  the  velocity  of  the 
observer  the  wave-front  of  an  emitted  beam  of  light  is  al- 
ways a  sphere  with  the  observer  as  the  center.  According 
to  the  older  views,  a  definite  point  of  the  ether,  the  source 
of  light,  was  the  center  of  the  sphere,  but  according  to  the 
newer  ideas  it  is  the  observer  himself  who  is  the  center  of 
the  sphere.  The  observer  is  thus  in  a  sense  the  center  of 
the  universe,  he  is  at  rest  and  the  universe  is  carried  by  for 
his  inspection.  If  an  observer  A  is  the  center  of  a  sphere, 
the  wave-front  will  have  advanced  after  time  t  so  that  the 
square  of  the  radius  of  the  sphere  will  be  represented  by 

tf  +  V2  +  Z2  =  C2/2 

or     x1  +  f  +  z2-  c2P  =  o 

where  the  observer  is  the  origin  of  coordinates  and  c  is  the 
velocity  of  light.  For  any  other  observer  B,  the  wave-front 

1  Eddington,  Contemporary  Review,  116,  643,  1919. 


436  ECLIPSES  OF  THE  SUN 

will  again  be  a  sphere  but  with  B  as  its  center,  and  the  mathe- 
matical equation  will  be 

x*  +  /  +  z'2  -  c2/'2  =  o 

According  to  the  relativity  assumption,  the  sphere  observed 
by  A  is  identical  with  that  observed  by  B.  Hence  a  linear 
transformation  between  jc,  yy  z,  t  and  x',  yf,  z'  y  t'  must  trans- 
form either  of  the  above  equations  into  the  other.  If  now 
we  put  let  =  w  (where  I  is  the  imaginary  quantity,  V  —  J)> 
then  the  equations  reduce  to  x1  -f~  T  +  z2  +  w2  =  o  and 
x'~  +  y's  +  %'*  +  w'~  =  °>  eac^  °f  which  represents  a 
sphere  in  four-dimensional  space,  x,  y,  z,  w  form  a  set  of  or- 
thogonal coordinates,  and  x',  y',  z',  w'  must  represent  a 
second  set  of  orthogonal  axes  in  this  same  space.  In  space  of 
three  dimensions  we  are  familiar  with  such  sets  of  axes.  An 
observer  at  any  one  place  chooses  three  axes,  one  north  and 
south,  the  second  east  and  west  and  the  third  vertically  up 
and  down.  Another  observer  at  a  different  place  on  the 
surface  of  the  earth  takes  a  similar  set  of  axes  with  reference 
to  his  directions  of  north,  east  and  vertical.  But  what  are 
up  and  down,  and  north  and  south  to  one  observer  are  not 
the  same  directions  that  appear  to  the  other  observer.  A 
rotation  of  axes  and  a  transfer  of  the  origin  will  transform 
the  set  of  axes  of  one  observer  into  those  of  the  other. 
Similarly  in  the  four-dimensional  space-time  continuum  the 
coordinates  of  A  can  be  transformed  into  those  of  B  by  a 
rotation  of  axes.  But  the  sphere  observed  by  A  being  the 
same  as  that  observed  by  B,  or  by  any  other  observer,  it 
therefore  must  appear  as  an  objective  reality.  But  A  and  B 
divide  up  the  space-time  continuum  differently.  Space  and 
time  depend  only  on  the  consciousness  of  the  individual  ob- 
server; hence  the  coordinates  that  A  calls  pure  space  and 
pure  time  will  not  be  space  and  time  to  observer  B  but  will 
be  a  mixture  of  the  two.  It  is  only  the  combination  of 
space  and  time,  not  space  and  time  separately,  that  repre- 
sents reality  independent  of  the  peculiarities  of  the  indi- 
vidual observer.  The  mathematical  expressions  by  means 
of  which  x,  y,  z,  w  are  rotated  into  x',  y',  zfy  w'  are  the 


EINSTEIN  THEORY  OF  RELATIVITY  437 

famous  transformations  of  Lorentz  which  were  derived  a 
decade  before  Einstein's  work  was  published. 

Starting  with  these  transformations  and  assuming  that 
light  follows  a  geodesic  and  not  a  straight  line  (which  is 
now  a  meaningless  term),  Einstein  developed  his  famous 
theory  of  gravitation.  The  only  constant  introduced  into 
the  discussion  is  c  the  velocity  of  light,  the  maximum  velocity 
of  which  we  have  any  observational  experience.  Virtually 
the  only  assumption  made  is  the  one  of  fundamental  im- 
portance, that  the  velocity  of  light  is  identical  to  all  ob- 
servers. This  is  at  best  an  assumption,  and  if  it  can  be 
shown  by  any  experiments  that  this  premise  is  not  true,  then 
the  whole  Einstein  structure  will  fall  into  ruins.  By  means 
of  a  little-used  and  very  difficult  branch  of  mathematics, 
a  theory  was  developed  which  explains  gravitation,  the 
motion  of  Mercury  and  the  gravitational  attraction  of  the 
sun  on  light  rays.  As  the  author  does  not  claim  to  be  one 
of  the  original  dozen  who  could  understand  Einstein's  equa- 
tions he  will  not  introduce  any  further  mathematical  formu- 
las. These  newer  conclusions  will  herald  an  advance  over 
the  old  law  of  gravitation  only  on  the  condition  that  Ein- 
stein's theory  represents  the  observed  facts  more  closely 
than  the  theory  of  Newton. 

We  may  perhaps  obtain  an  inkling  of  the  meaning  of 
four-dimensional  space  by  starting  out  with  the  properties 
of  three-dimensional  space,  with  which  from  childhood  we 
have  been  familiar.  In  the  world  of  events  all  four  coor- 
dinates are  necessary,  for  we  never  observe  an  event  except 
at  a  certain  time,  and  we  are  never  cognizant  of  an  instant 
of  time  without  special  reference  to  space.  In  discussing 
the  laws  of  electromagnetic  phenomena,  it  was  shown  by 
Minkowski  that  these  laws  could  be  represented  by  assum- 
ing a  four-dimensional  space-time  and  that  the  mathematical 
transformations  of  Lorentz  and  Einstein  could  be  described 
by  a  rotation  of  a  set  of  axes.  In  three-dimensional  space 
of  everyday  happenings  no  one  would  be  impertinent  enough 
to  say  that  there  is  any  absolute  direction  of  up  and  down, 
i.e.,  the  vertical,  at  any  one  place  on  the  earth's  surface, 
although  there  are  great  numbers  of  people  who  believe  that 


438  ECLIPSES  OF  THE  SUN 

New  York  City  occupies  a  specially  important  place  on  the 
top  of  the  world.  The  words  "  sooner  or  later  "  are  like- 
wise relative  terms  only  and  not  absolute.  We  are  all  fa- 
miliar with  the  mental  process  of  assuming  a  set  of  three 
rectangular  axes  relating  to  the  particular  location  of  the 
observer  on  the  earth's  surface,  a  set,  therefore,  of  relative 
coordinates,  not  absolute  ones.  If  we  make  a  section  by  a 
plane  of  three-dimensional  space,  we  derive  a  two-dimen- 
sional space,  a  plane.  Similarly,  if  a  section  is  made  through 
the  four-dimensional  world  of  events,  a  three-dimensional 
space  will  result.  For  an  observer  on  the  earth,  there  is  one 
particular  section  only  which  will  give  the  space  of  three 
dimensions  with  which  he  is  familiar,  so  that  the  four-dimen- 
sional unity  thus  breaks  up  for  him  into  space  and  ordinary 
time.  A  section  through  an  observer  on  Mars  would  be 
unreal  to  us  and  unnatural  in  aspect.  One  of  the  postulates 
regarding  four-dimensional  space  is  that  it  must  be  entirely 
independent  of  the  observer  so  that  there  cannot  be  any 
real  difference  between  any  two  directions  in  an  absolute 
sense.  For  any  particular  observer,  as  we  have  seen,  the 
four-dimensional  world  of  events  may  break  up  into  ordi- 
nary space  and  time,  but  such  a  section  would  represent  a 
particular  case  and  not  the  most  general  solution. 

We  may  draw  a  number  of  lines  on  a  flat  piece  of  paper 
or  sheet  of  rubber.  These  lines  intersect  in  many  points. 
By  taking  the  sheet  of  rubber  into  our  hands  and  altering 
its  shape,  the  lines  on  its  surface  become  curves  in  three- 
dimensional  space.  A  skillful  mathematician  could  repre- 
sent these  curves  by  complicated  equations  of  transforma- 
tions involving  x,  y  and  z.  By  altering  the  shape  of  the 
piece  of  rubber  we  change  the  appearance  of  the  lines  on  its 
surface,  but  we  do  not  in  the  least  change  the  number  of  the 
intersections  made  by  the  lines.  The  transformations  ex- 
pressing the  intersection  of  two  lines  may  vary  greatly  in 
their  mathematical  forms,  but  if  they  are  true  formulas,  they 
cannot  alter  the  actual  intersections  of  the  lines  themselves. 
A  simple  case  of  change  of  axes  of  reference,  a  mathemati- 
cal transformation,  is  familiar  to  everyone.  While  standing 
out  in  the  rain,  when  no  wind  is  blowing,  one  holds  his  um- 


EINSTEIN  THEORY  OF  RELATIVITY  439 

brella  vertically  over  his  head  if  he  wishes  to  keep  dry. 
The  rain-drops,  obeying  the  law  of  gravitation,  seem  to  fall 
vertically.  If  the  observer  starts  to  walk,  the  direction  of 
the  falling  rain  appears  to  have  changed,  the  rain-drops 
come  in  a  slanting  direction.  Of  course,  the  direction  has 
not  actually  changed,  but  the  effect  is  just  the  same  as  if 
there  had  been  a  real  change  in  the  direction  of  gravity. 
Some  obstinate  person  will  say  that  he  knows  that  gravity 
is  acting  in  a  vertical  direction  and  that  his  motion  has  noth- 
ing at  all  to  do  with  gravity.  Under  this  assumption  he 
persists  in  holding  the  handle  of  his  umbrella  vertical,  for 
he  has  learned  that  when  standing  at  rest  he  keeps  dry  by 
so  doing.  If  he  still  insists  in  his  foolishness  when  going  at 
high  speed  in  a  swift-moving,  open  automobile,  he  will  lose 
his  umbrella  and  get  dripping  wet  into  the  bargain. 

In  four-dimensional  space  the  "  world-lines  "  intersect  in 
a  series  of  events.  For  the  description  of  the  world-line  we 
can  choose  any  set  of  four-dimensional  axes  we  wish.  The 
event  takes  place,  however,  absolutely  independently  of 
any  assumption  of  reference  axes.  If  we  change  the  axes, 
we  at  the  same  time  change  the  coordinates  with  respect  to 
these  axes,  but  the  interesting  thing  is  the  event  and  not 
the  particular  set  of  reference  axes  assumed.  The  world  of 
nature  is  made  known  to  us  only  through  a  series  of  obser- 
vations or  series  of  events.  We  learn,  in  fact,  only  through 
a  series  of  coincidences.  To  represent  the  laws  of  nature 
our  observations  must  be  true  no  matter  what  selection  we 
make  of  reference  axes.  In  other  words,  the  mathematical 
expression  of  the  laws  of  nature  must  be  such  that  their  form 
does  not  change  if  we  make  a  transformation  of  axes. 

It  is  a  curious  fact  that  we  know  very  little  of  the  mecha- 
nism by  which  gravitation  works.  Is  it  propagated  with  the 
velocity  of  light,  or  does  it  act  instantaneously  at  infinite 
speed?  At  a  place  on  the  earth's  surface,  gravity  is  the 
resultant  of  two  forces,  one  the  attraction  of  the  earth,  the 
other  due  to  the  centrifugal  force  of  the  earth's  rotation. 
The  former  is  spoken  of  generally  as  a  "  natural  "  force, 
the  latter  as  an  "  artificial  "  one.  Centrifugal  force  may  be 
altered  by  taking  different  locations  on  the  surface  of  the 


440  ECLIPSES  OF  THE  SUN 

earth  or  may  be  made  to  vanish  entirely  by  going  to  the 
north  or  south  poles.  For  the  purpose  of  simplicity  in  the 
mathematical  treatment,  centrifugal  force  is  separated  from 
gravity.  We  have  no  direct  sensation  for  either  force  sepa- 
rately, and  in  fact  there  is  no  physical  basis  for  the  separa- 
tion. So  why  not  consider  the  "  natural  "  and  the  "  arti- 
ficial "  force  on  the  same  basis?  The  generalization  of  this 
notion  led  to  Einstein's  principle  of  equivalence  that  a 
gravitational  field  of  force  is  precisely  equivalent  to  an 
artificial  field  of  force,  so  that  it  is  impossible  by  any  con- 
ceivable experiment  to  distinguish  between  them.  Force, 
therefore,  is  relative  and  not  absolute. 

Guided  by  the  two  principles  of  relativity  and  equivalence, 
Einstein  was  led  to  assume  that  all  gravitational  fields  of 
force  must  be  illusions.  He  himself  admits  that  he  was 
brought  to  his  new  point  of  view  by  discussing  the  sensa- 
tions of  falling  with  a  man  who  had  just  tumbled  from  a 
high  building.  No  distinct  consciousness  of  falling  was 
actually  experienced  by  the  man.  The  simplest  method 
after  all  for  the  consideration  of  gravitation  may  actually 
be  the  point  of  view  of  the  falling  man  who  was  experiencing 
no  peculiar  sensations  of  his  sudden  flight.  This  was 
Einstein's  method.  The  earth  was  surely  rising  to  meet 
the  man,  and  if  he  had  not  known  of  the  sudden  stoppage 
of  his  flight  that  was  in  store  for  him,  he  might  have  quietly 
theorized  regarding  the  relativity  of  motion.  Such  an  un- 
constrained body  (as  the  falling  man)  if  left  to  itself  takes 
what  we  shall  call  a  "  straight  "  line,  but  which  is  actually 
a  geodesic.  The  world-line  is  undeflected  until  the  particle 
of  matter  comes  into  the  vicinity  of  another  particle.  The 
world-line  of  each  particle  is  bent  in  towards  each  other  or 
deformed.  A  change  of  direction  of  the  particle  means  that 
a  field  of  force  has  been  entered.  Such  a  deformation  can 
be  brought  into  the  equations  by  means  of  mathematical 
transformations. 

Gravitation  therefore  needs  no  special  treatment  different 
from  that  of  any  other  force.  In  developing  the  mathe- 
matical analysis  it  was  not  possible  to  follow  the  geometry 
of  the  flat  space  of  Euclid  where  the  straight  line  is  the 


EINSTEIN  THEORY  OF  RELATIVITY 

shortest  distance  between  two  points.  The  mathematician 
will  assert  that  there  is  no  reason  to  assume  Euclidian 
geometry  unless  observation  demands  it.  In  the  process  of 
the  difficult  mathematical  development,  Einstein  was  at 
liberty  to  choose  between  several  possibilities,  and  the  deci- 
sion reached  seemed  to  give  the  simplest  solution.  In  the 
final  analysis  his  law  of  gravitation  must  produce  the  same 
effect  as  Newton's  law  for  conditions  of  velocities  small 
compared  with  that  of  light. 

According  to  Einstein's  views,  gravitation  is  simply  a 
geometrical  deformation  of  unrestrained  bodies,  and  hence 
we  may  regard  the  gravitational  field  as  influencing  or  even 
determining  the  laws  of  space-time  derived  from  measure- 
ments. The  track  of  a  ray  of  light  therefore  is  deformed 
by  the  gravitational  field.  According  to  Eddington  1  "  a 
ray  of  light  passing  near  a  heavy  particle  will  be  bent, 
firstly,  owing  to  the  non-Euclidian  character  of  the  com- 
bination of  time  with  space.  This  bending  is  equivalent  to 
that  due  to  the  Newtonian  gravitation,  and  may  be  cal- 
culated in  the  ordinary  way  on  the  assumption  that  light 
has  weight  like  a  material  body.  Secondly,  it  will  be  bent 
owing  to  the  non-Euclidian  character  of  space  alone,  and 
this  curvature  is  additional  to  that  predicted  by  Newton's 
law.  If  then  we  can  observe  the  amount  of  curvature  of  a 
ray  of  light,  we  can  make  a  crucial  test  of  whether  Einstein's 
or  Newton's  theory  is  obeyed. 

"  This  separation  of  the  attraction  into  two  parts  is  useful 
in  a  comparison  of  the  new  theory  with  the  old,  but  from 
the  point  of  view  of  relativity  it  is  artificial.  Our  view  is 
that  light  is  bent  in  just  the  same  manner  as  the  track  of  a 
material  particle  moving  with  the  same  velocity  would  be 
bent.  Both  causes  of  bending  may  be  ascribed  either  to 
weight  or  to  non-Euclidian  space-time,  according  to  the 
nomenclature  preferred.  The  only  difference  between  the 
predictions  of  the  old  and  new  theories  is  that  in  one  case 
the  weight  is  calculated  according  to  Newton's  law  of  gravi- 
tation, in  the  other  case  according  to  Einstein's." 

It  has  been  repeatedly  urged  by  many  writers  that  New- 

1  Spacey  Time  and  Gravitation,  p.  106. 


442  ECLIPSES  OF  THE  SUN 

ton's  law  is  simpler  than  Einstein's.  The  former  is  indeed 
more  familiar,  but  that  does  not  necessarily  mean  that  it  is 
simpler.  This  depends  on  the  point  of  view.  The  principle  of 
relativity  introduced  into  scientific  thought,  first  destroyed 
the  notion  that  space  and  time  were  absolute  in  character 
or  objective  in  existence.  This  led  to  a  consideration  of  the 
four-dimensional  world  of  events  and  to  the  supposition  that 
gravitational  force  is  an  illusion.  To  explain  this  "  force  " 
as  an  inherent  curvature  in  space  represents  a  further 
revolution  in  scientific  thought.  Gravitation  is  not  the  only 
"  force  "  familiar  to  the  physicist.  Are  these  also  illusions 
and  must  not  the  Einstein  theory  be  generalized  to  include 
all  of  them?  Such  a  generalization  has  been  proposed  by 
Weyl  who  of  necessity  was  forced  to  introduce  new  curva- 
tures in  the  four-dimensional  space  to  represent  the  forces 
involved ;  in  fact,  the  effects  predicted  by  Weyl  agree  so  per- 
fectly with  electromagnetic  theory  that  no  testing  of  the 
theory  by  experiment  is  possible.  The  only  scientific  test 
to  which  all  theories  and  laws  must  submit  is  the  funda- 
mental one  of  whether  they  represent  the  observed  facts. 
Einstein  has  not  attempted  an  explanation  of  gravitation 
other  than  to  assume  that  it  is  propagated  with  the  velocity 
of  light;  he  has  only  been  occupied  with  the  deduction  of  its 
laws. 

In  the  foregoing  chapter  is  given  an  inkling  of  the  man- 
ner in  which  the  philosophic  trend  of  thought  has  been  modi- 
fied by  the  theory  of  relativity;  in  the  following  pages  some 
few  of  the  scientific  consequences  will  be  discussed. 


CHAPTER  XXIII 

HAS  THE  EINSTEIN  THEORY  BEEN  VERIFIED? 

IF  the  path  of  a  ray  of  light  is  identical  with  that  of  a 
material  particle  when  passing  close  to  a  body  of  heavy 
mass  like  the  sun,  it  is  easy  to  see  that  the  path  will  not 
be  straight  but  will  be  a  hyperbola.    To  calculate  the  amount 
of  the  deflection  from  the  straight  line  course  one  might 
apply  Newton's  law;  but  according  to  Einstein's  theory,  this 
old  and  well  established  law  is  applicable  only  under  the  con- 
dition of  small  velocities.    For  an  object  traveling  with  the 
velocity  of  light,  the  Einstein  formula  gives  a  deflection 
twice  that  deduced  from  Newton's  law. 

Other  important  consequences  follow  as  a  direct  result 
of  the  Einstein  theory.  According  to  the  inverse-square 
law  of  gravitation,  an  isolated  planet  will  describe  about 
the  sun  an  ellipse  the  direction  of  whose  axis  is  fixed  in  space, 
but  according  to  the  Einstein  law  the  path  will  be  a  spiral 
and  not  an  ellipse.  The  direction  of  the  major  axis  will 
therefore  rotate  in  space  instead  of  being  stationary.  The 
amount  of  rotation,  calculated  from  the  Einstein  theory  and 
derived  without  the  introduction  of  any  additional  constants 
into  the  equations,  is  43 "  per  century  for  the  planet 
Mercury. 

A  further  consequence  is  found  when  comparing  the  time 
of  vibration  of  an  atom  acting  under  the  strong  gravita- 
tional pull  of  the  sun  with  that  of  a  similar  atom  in  a  ter- 
restrial laboratory.  The  atom  in  its  vibrations  always  be- 
haves very  much  like  the  pendulum  of  an  ideal  clock.  Since 
the  gravitational  field  of  force  is  similar  in  its  properties  to 
that  of  a  centrifugal  field,  it  is  comparatively  easy  to  calcu- 
late the  effect  of  the  theory  of  relativity  on  the  vibrations 
of  light  under  the  gravitational  action  of  the  sun.  It  has 

443 


444  ECLIPSES  OF  THE  SUN 

been  found,  in  virtue  of  this  theory,  that  the  wave-lengths 
at  the  solar  surface  should  be  greater  than  those  under  ter- 
restrial conditions  in  the  ratio  of  1.00000212:1.  Each  line 
in  the  solar  spectrum  should  be  shifted  towards  greater 
wave-lengths,  or  towards  the  red  end  of  the  spectrum,  by 
an  amount  readily  calculable.  The  shift  at  the  F  or  H(3  line 
in  the  blue-green  part  of  the  solar  spectrum  should  amount 
to  o.o  10  A,  or  one-hundreth  of  an  angstrom. 

There  is  a  fourth  consequence  of  the  Einstein  theory.  If 
a  massive  particle  is  placed  near  the  center  of  a  circle  and  if 
the  length  of  the  circumference  and  the  length  of  the  diam- 
eter are  accurately  measured,  it  should  be  found  that  these 
two  lengths  will  not  be  in  the  ratio  of  the  famous  number 
TT.  The  experiment  can  hardly  be  regarded  as  a  crucial 
one  for  testing  the  Einstein  theory  for  the  reason  that  a 
weight  of  five  tons  placed  inside  a  circle  of  five  yards  radius 
would  not  change  the  value  of  ir  by  more  than  one  figure  in 
the  25th  place  of  decimals. 

The  astronomers  at  the  eclipse  of  1919  observed  deflec- 
tions in  the  rays  of  light  from  the  stars  in  the  neighborhood 
of  the  sun  apparently  in  complete  harmony  with  the  pre- 
diction of  Einstein.  As  already  stated,  the  amount  of  bend- 
ing predicted  was  i".75  for  a  beam  just  grazing  the  edge  of 
the  sun.  Was  the  observed  deflection  an  actual  verification 
of  the  Einstein  theory  or  can  it  be  accounted  for  by  some 
other  agency?  After  eliminating  deflections  from  all  known 
causes,  the  observations  must  necessarily  conform  to  one 
of  three  possibilities.  First,  there  may  be  no  deflections 
whatever  in  the  stellar  rays,  light  traveling  in  exact 
straight  lines  under  all  possible  conditions;  second,  the 
amount  may  be  the  so-called  Newtonian  deflection  of  o".8 7; 
or  third,  it  may  be  twice  this  size,  as  predicted  by  Einstein, 
i.e,,  a  curvature  of  space  in  the  vicinity  of  the  sun  causing 
a  bending  of  a  light  ray  equal  in  amount  to  that  demanded 
by  the  Newtonian  theory.  There  are  other  deflections  pos- 
sible, but  they  need  not  be  considered  in  the  present  status 
of  the  scientific  theory. 

Several  books  and  articles  have  recently  been  written 
claiming  that  the  gravitational  pull  of  the  sun  has  absolutely 


EINSTEIN  THEORY  VERIFIED?  445 

no  effect  on  a  beam  of  light.  The  authors  confidently  state 
that  observation  has  abundantly  verified  the  conclusion  that 
light  travels  always  in  perfect  straight  lines  and  hence  a 
deflection  of  light  rays  by  the  sun  is  impossible.  The  brave, 
or  possibly  foolhardy,  writers  apparently  have  failed  to  real- 
ize that  with  one  fell  swoop  they  utterly  disregard  not 
only  the  whole  of  the  theory  of  relativity  but  at  the  same 
time  they  deny  that  light  has  weight.  The  consequences  of 
relativity  have  been  so  thoroughly  substantiated  by  observa- 
tions that  he  who  has  a  scientific  reputation  at  stake  must 
indeed  be  very  rash  to  state  that  physicists  must  all  be  mis- 
taken and  that  the  whole  theory  of  relativity  is  "  tommy- 
rot^ 

That  light  has  weight  has  been  thoroughly  and  completely 
verified.  Clerk  Maxwell  showed  many  years  ago  that  light 
is  electromagnetic  in  character.  In  fact,  the  waves  produc- 
ing X-rays,  photographic  light,  visible  light,  heat  and  wire- 
less waves,  are  exactly  similar  in  character,  the  only  real 
difference  being  the  length  of  the  waves.  The  wave-lengths 
of  X-rays  are  ten  thousand  times  smaller  than  those  of 
light  rays  while  wireless  waves  are  just  as  much  larger. 
Light  unquestionably  possesses  mass  or  inertia  like  other 
forms  of  electromagnetic  energy.  The  tiny  waves  of  light 
exert  a  pressure  on  material  objects  in  a  manner  quite  simi- 
lar to  that  manifested  by  waves  in  water  that  pound  with 
such  terrific  force  on  the  sea  shore  at  times  of  storm.  The 
pressure  exerted  by  light  is  termed  light-pressure  or  radia- 
tion-pressure. Its  consequences  are  described  in  Chapter 
XXI,  and  its  effects  are  manifested  to  the  astronomer  in  the 
solar  corona,  in  the  tails  of  comets  and  in  the  conditions  in 
the  more  diffuse  giant  stars.  This  pressure  exerted  by  light 
has  been  found  experimentally  in  the  physical  laboratory. 
It  is  a  consequence  of  the  orthodox  electromagnetic  theory. 
Recent  discoveries,  outlined  in  Chapter  XIV,  have  shown 
that  mass  is  electrical  in  character,  and  consequently  energy 
is  likewise  electrical,  and  hence  light  which  is  also  electrical 
must  possess  both  mass  and  energy.  If  any  writer  denies 
these  conclusions,  he  does  not  by  so  doing  destroy  the  sci- 
entific facts,  but  succeeds  only  in  displaying  his  own  igno- 


446  ECLIPSES  OF  THE  SUN 

ranee  of  these  facts.  Even  Newton  himself  surmised  that 
light  has  weight.  The  admirers  of  the  great  Englishman 
should  therefore  not  dogmatically  deny  proven  scientific 
knowledge  and  state  that  light  moves  under  all  circumstances 
in  straight  lines  even  when  under  the  strong  gravitational  at- 
traction of  the  sun.  A  beam  of  light  should  therefore  be  de- 
flected from  its  straight  line  course  like  any  material  particle 
moving  with  the  same  velocity,  and  hence,  in  interpreting 
the  results  of  the  eclipse  photographs  we  must  rule  out  the 
possibility  that  no  deflection  can  be  caused  by  the  sun.  But 
which  is  the  true  deflection,  the  Newtonian  or  that  predicted 
by  Einstein? 

The  only  possible  method  of  testing  the  relative  merits  of 
the  two  theories  observationally  is  by  means  of  photograph- 
ing the  stars  surrounding  the  sun.  On  account  of  the  glare 
of  the  daylight  sky,  it  has  been  found  impossible  to  photo- 
graph the  stars  in  the  immediate  vicinity  of  the  sun,  at  any 
time  except  when  the  sun  is  totally  eclipsed.  The  amount 
of  the  deflection  depends  on  the  angular  distance  of  the  star 
from  the  sun's  edge,  and  at  a  distance  of  one  solar  radius 
the  angular  deflection  is  but  half  the  value  at  the  limb.  At 
a  distance  of  a  solar  diameter,  the  deflection  is  but  one-third 
of  the  limb  value.  The  most  important  stars  for  the  purpose 
of  testing  the  theory  are  therefore  those  which  are  closest 
to  the  sun.  Naturally,  the  brighter  the  star  the  more  readily 
can  it  be  photographed.  What  is  needed  for  a  test  of  the 
theory  is  a  group  of  bright  stars  distributed  at  various  angles 
around  the  sun  and  close  to  its  edge.  Since  the  sun  is 
always  found  in  the  plane  of  the  ecliptic,  it  is  easy  to  see  at 
a  glance  from  a  celestial  chart  which  part  of  the  ecliptic  con- 
tains the  brightest  groups  of  stars,  and  then  to  decide  at 
what  date  of  the  year  the  sun  is  found  at  this  favorable 
position.  The  date  turns  out  to  be  May  29.  The  brightest 
assemblage  of  ecliptic  stars  is  the  group  known  as  the 
Hyades  in  the  constellation  of  Taurus. 

The  British  observers  in  1919  were  especially  favored,  for 
the  eclipse  occurred  on  the  twenty-ninth  of  May.  Two  ex- 
peditions were  dispatched.  Crommelin  and  Davidson  repre- 
senting the  Greenwich  Observatory  went  to  Sobral  in  North 


EINSTEIN  THEORY  VERIFIED?  447 

Brazil^  while  Eddington  and  Cottingham  from  Cambridge 
located  at  the  Isle  of  Principe  in  the  Gulf  of  Guinea,  West 
Africa.  The  telescope  used  at  Principe  was  the  thirteen-irich 
"  astrographic  "  telescope  belonging  to  the  Oxford  Observa- 
tory. The  telescope  was  used  horizontally,  the  light  from  the 
sun  being  reflected  down  the  tube  by  a  coelostat  mirror.  The 
focal  length  of  1 1  feet  4  inches  insures  a  scale  of  the  photo- 
graphs of  one  millimeter  equal  to  one  minute  of  arc.  One 
second  of  arc  is  thus  equal  to  one-sixtieth  of  a  millimeter 
or  one-fifteenth-hundreth  of  an  inch.  In  order  to  secure 
increased  sharpness  of  the  star  images,  the  aperture  of  the 
lens  was  stopped  down  to  eight  inches.  On  eclipse  day 
clouds  greatly  interfered  with  the  progress  of  the  work.  On 
account  of  the  long  duration  of  totality  amounting  to  six 
minutes,  sixteen  photographs  were  obtained  with  exposures 
ranging  from  2  to  20  seconds.  Unfortunately  on  many  of 
the  plates,  one  or  more  of  the  essential  stars  were  missing  on 
account  of  clouds. 

In  Brazil  the  observers  were  favored  with  fine  weather. 
They  utilized  two  telescopes  for  securing  their  photographs, 
one  similar  in  size  to  the  one  at  Principe,  the  other  with  the 
greater  focal  length  of  19  feet  and  aperture  four  inches. 
The  photographs  with  the  former  instrument  were  a  grave 
disappointment,  for  the  star  images  were  not  in  sharp  focus, 
due  unquestionably  to  the  warping  of  the  mirror  by  the  heat 
of  the  sun.  Seven  of  the  plates  taken  with  the  four-inch  lens 
were  found  satisfactory  for  measurement.  Only  two  of  the 
Principe  plates  were  measured  and  on  these  the  images  were 
not  particularly  well-defined  and  moreover  they  were  weak 
due  to  the  interposing  clouds.  The  sixteen  plates  secured  at 
Sobral  with  the  astrographic  telescope  were  also  measured 
in  spite  of  their  poor  definition. 

In  order  to  secure  from  the  eclipse  plates  results  of  the 
highest  value,  it  is  necessary  to  photograph  as  a  check  the 
same  region  in  the  sky  but  with  the  sun  absent.  To  be 
of  the  greatest  worth,  the  check  plates  thus  obtained  should 
reproduce  as  closely  as  possible  the  conditions  under  which 
the  eclipse  photographs  are  taken.  The  plates  should,  if 
possible  be  taken  at  the  site  of  the  eclipse,  and  with  hour 


448  ECLIPSES  OF  THE  SUN 

angle,  temperature,  etc.,  of  the  eclipse  duplicated  as  nearly 
as  possible  in  the  check  photographs.  Such  a  procedure 
requires  a  stay  of  two  or  three  months  at  the  eclipse  camp, 
frequently  located  thousands  of  miles  from  home.  The 
Principe  observers  indeed  secured  check  plates,  but  not  at 
Principe,  but  with  the  telescope  mounted  in  England.  The 
conditions  of  latitude  and  temperatures  were  vastly  differ- 
ent in  the  two  series  of  plates,  and  furthermore  the  plates 
taken  in  England  were  secured  without  the  interposition  of 
the  mirror  used  at  eclipse  time.  In  view  of  their  faulty 
nature  as  check  plates,  an  additional  precaution  was  taken 
of  securing  an  extra  series  of  plates  photographed  at  Prin- 
cipe and  in  England  but  portraying  a  different  stellar  region, 
since  it  was  found  impractical  to  remain  at  Principe.  The 
Sobral  observers  did  remain  an  additional  two  months  after 
the  eclipse  to  secure  the  desired  check  plates. 

The  values  of  the  Einstein  effect  reduced  to  tangency  to 
the  sun's  limb  are: 

From     7  Sobral  Plates  taken  with  4-inch  lens,   i".98=bo".i2 
From     2  Principe     "  "          "     13  "       "       i"6i±o".3O 

From   16  Sobral       "  "         "     13  "       "       o".86dbo".i 

On  account  of  the  poor  definition  of  the  sixteen  plates 
taken  with  the  Sobral  1 3-inch  lens,  the  values  of  the  Einstein 
deflection  from  these  plates,  ranging  as  they  did  from  o".oo 
to  i ".2  8,  were  omitted  from  the  final  mean.  Combining  the 
two  remaining  series  with  weights  depending  on  the  prob- 
able errors,  there  results  the  deflection  of  i".Q3.  Giving 
equal  weight  to  each  of  the  series,  the  value  becomes  i".8o. 
The  value  predicted  by  Einstein  is  i".J5.  The  close  agree- 
ment between  observation  and  prediction  was  received  with 
the  greatest  interest  and  remarkable  enthusiasm  throughout 
the  whole  of  the  thinking  world. 

The  best  series  of  the  1919  plates  gives  a  result  i".g&  ± 
o".i2.  This  certainly  does  not  appear  to  confirm  the  half- 
deflection  of  0^.87,  but  on  the  other  hand  the  poorest  of  the 
three  series  of  plates  does  furnish  a  value  closely  agreeing 
with  the  Newtonian  deflection.  The  remaining  value, 
i ".6 1  =t  o".30,  although  it  agrees  closely  with  the  full  de- 


8:9 


EINSTKIN  THEORY  VERIFIED?  449 

flection,  might,  on  account  of  the  size  of  the  probable  error, 
possibly  confirm  the  half  amount.  Although  the  plates  taken 
at  the  1918  eclipse  by  the  Lick  Observatory  with  the  five- 
inch  and  forty-feet  focus  camera  pointed  directly  at  the  sun 
gave  no  indication  of  any  appreciable  bending,  the  notion 
that  light  must  necessarily  move  in  straight  lines  was  now 
forever  gone  from  scientific  thought. 

In  spite  of  the  general  good  agreement  of  the  photographic 
measures,  no  one,  least  of  all  the  British  observers  them- 
selves, will  affirm  that  the  Einstein  deflection  had  been  com- 
pletely verified  by  the  1919  eclipse  results.  The  work  was 
pioneer  in  character;  and  improvements  over  the  methods 
followed  were  readily  apparent.  First  of  all,  the  coelostat 
mirrors  must  be  eliminated  by  placing  the  cameras  on  an 
equatorial  mounting  and  pointing  the  lenses  directly  at  the 
sun.  Not  only  do  the  mirrors  prevent  a  maintenance  of 
constant  focus,  on  account  of  the  warping  of  their  surfaces 
by  the  heat  of  the  sun,  but  the  mirrors  may  introduce  dis- 
tortions in  the  photographs.  As  Russell,  Slocum  and  others 
have  proved,  evidences  of  such  distortions  are  shown  even 
in  the  published  results  of  the  British  observers.  It  was 
immediately  recognized  that  in  the  future  much  greater  care 
must  be  exercised  in  securing  photographs  of  the  field  of  stars 
in  the  midst  of  which  the  sun  would  be  found  on  the  day  of 
the  eclipse.  The  conditions  under  which  these  plates  are 
obtained  must  resemble  as  closely  as  possible  those  at  the 
time  of  the  eclipse,  i.e.,  with  same  temperature,  hour  angle 
of  telescope,  etc.  Scientific  men  are  fully  aware  of  the 
fact  that  errors  may  creep  into  measures  no  matter  how 
carefully  made  and  every  effort  must  be  made  to  minimize 
these  errors  or  to  eliminate  them  if  possible.  The  chief  place 
of  importance  at  the  1922  eclipse  was  given  to  this  problem. 

It  must  not  be  thought  that  the  amount  of  the  deflection 
caused  by  the  sun  is  an  angle  so  minute  in  size  that  its  meas- 
urement will  tax  the  resources  of  modern  astronomy.  In 
determining  the  distances  of  the  fixed  stars,  parallal  ob- 
servers are  quite  used  to  measuring  displacements  on  their 
photographic  plates  less  than  one-hundredth  of  the  Einstein 
deflection  by  the  sun.  There  are  half  a  dozen  of  the  observa- 


450  ECLIPSES  OF  THE  SUN 

tories  in  the  United  States  alone  which  are  devoting  much 
of  their  energies  to  parallax  determinations.  Nor  does  the 
problem  of  measuring  the  eclipse  plates  contain  anything 
startlingly  new  in  the  method.  Astronomers  for  many  years 
are  quite  familiar  with  all  of  the  processes  involved  in  the 
measurements  and  reductions,  and  moreover  they  are  quite 
accustomed  to  securing  an  accuracy  far  greater  than  that  de- 
manded by  the  Einstein  problem.  In  regular  observatory 
work,  such  as  required  in  the  determination  of  the  parallax 
of  the  stars,  astronomers  are  confident  that  constancy  of  re- 
sults can  be  expected  from  night  to  night  in  photographs 
taken  with  the  telescope,  and  they  have  long  series  of  observa- 
tions to  prove  their  conclusions;  in  fact,  in  parallax  deter- 
minations an  accuracy  is  secured  which  is  measured  by  a 
probable  error  one  two-hundredth  part  of  the  maximum  de- 
flection demanded  by  the  Einstein  theory. 

If  a  total  eclipse  took  place  every  day,  the  astronomer 
would  have  abundant  opportunity  to  vary  the  observing  con- 
ditions and  thoroughly  eliminate  all  possible  sources  of  error, 
but  with  eclipses  occurring  so  seldom  he  must  do  the  best  he 
can  under  unusual  conditions.  The  check  photographs  are 
secured  for  this  purpose.  The  scale  of  the  photographs,  the 
orientation  and  the  various  correcting  factors  that  must  be 
taken  into  account  in  the  reductions  are  obtained  from 
the  check  photographs.  If  these  plates  are  photographed 
through  the  glass,  which  is  possible  by  the  use  of  plate  glass, 
then  the  eclipse  plates  and  the  check  plates  may  be  placed 
film  to  film  and  the  star  images  on  the  eclipse  plates  will 
nearly  match  in  position  those  of  the  check  plates.  The  meas- 
urement thus  becomes  a  differential  one  of  comparing  the 
stars  on  the  eclipse  plate,  image  by  image,  with  those  of  the 
check  plates.  The  method  of  reduction  makes  due  allowance 
for  any  changes  in  scale-value,  orientation,  refraction,  etc., 
and  we  can  confidently  expect  that  the  gravitational  deflec- 
tion will  be  readily  separated  from  all  other  effects,  provided 
care  is  taken  in  securing  a  satisfactory  series  of  plates  to  act 
as  checks.  Before  discussing  the  results  of  the  1922  eclipse 
we  must  survey  the  other  consequences  of  the  Einstein 
theory. 


EINSTEIN  THEORY  VERIFIED?  451 

Of  the  eight  major  members  of  the  solar  system,  Mercury 
is  the  exceptional  planet.  It  is  closest  to  the  sun  and  has 
the  greatest  orbital  motion,  it  has  the  smallest  mass,  its 
orbit  has  the  greatest  eccentricity  and  is  inclined  by  the 
greatest  angle  to  the  ecliptic.  Mainly  on  account  of  the 
great  eccentricity  and  high  inclination,  the  path  of  the  planet 
has  caused  considerable  worry  to  the  mathematical  astrono- 
mer. Mercury  is  never  seen  more  than  28°  from  the  sun 
and  it  is  therefore  only  visible  to  the  naked  eye  shortly  after 
sunset  or  before  sunrise  projected  on  a  strong  twilight  sky. 
With  a  telescope,  however,  the  planet  may  be  observed  in 
the  daylight  sky.  Tables  of  motion  of  Mercury  were  made 
by  Kepler  as  early  as  1627,  by  Halley  about  1680  and  by 
Lalande  a  hundred  years  later.  The  first  really  accurate 
tables  were  those  of  Leverrier  based  on  meridian  observa- 
tions made  at  the  Paris  Observatory  between  the  years  1801 
and  1842,  and  supplemented  by  observations  of  transits  of 
the  planet  across  the  face  of  the  sun  beginning  with  that 
of  1631.  Leverrier  discussed  the  observations  in  a  thor- 
oughly masterful  manner,  calculating  the  gravitational  at- 
tractions of  the  other  planets  for  Mercury  whereby  the  orbit 
was  perturbed.  It  was  found  by  him  that  the  observed  mo- 
tion of  the  axis  of  the  planet's  ellipse  was  moving  0^.38  per 
year,  or  38"  per  century,  in  excess  of  the  amount  calculated 
from  the  law  of  gravitation. 

Leverrier  discovered  several  possible  reasons  for  the  dis- 
crepancy, the  most  plausible  being  the  presence  of  a  planet, 
or  group  of  planets,  between  the  orbit  of  Mercury  and  the 
sun.  On  March  26,  1859,  Lescarbault  observed  such  a  planet 
in  transit  across  the  face  of  the  sun.  It  was  found  later  that 
Lescarbault 's  planet  did  not  follow  the  orbit  calculated  for 
it  by  its  discoverer  and  that  the  transits  predicted  by  him 
failed  to  materialize.  Unquestionably,  the  original  observa- 
tion and  those  of  many  additional  transits  of  other  planets 
supposed  to  have  been  seen  by  others  find  a  ready  explana- 
tion in  the  appearance  of  spots  on  the  face  of  the  sun.  The 
discovery  of  two  intra-Mercurial  planets  at  the  eclipse  of 
1878  (see  Chapter  IX)  has  likewise  never  been  verified. 

Two  decades  after  Leverrier  had  completed  his  work,  the 


452  ECLIPSES  OF  THE  SUN 

same  problem  was  attacked  by  Simon  Newcomb ;  and  natu- 
rally with  much  more  material  to  his  hands  than  was  avail- 
able earlier.  No  less  than  5421  observations  of  Mercury 
entered  into  the  discussion,  and  a  total  of  over  sixty  thou- 
sand into  the  research  on  the  motions  of  the  four  inner 
planets,  Mercury,  Venus,  Earth  and  Mars. 

The  observed  motion  of  the  perihelion  of  Mercury  de- 
duced by  Newcomb  was  4i".6±i".4  in  excess  of  that  cal- 
culated from  the  law  of  gravitation.  Such  a  large  discrep- 
ancy appearing  in  Mercury,  what  of  the  other  planets?  The 
orbit  of  Venus  is  really  too  nearly  circular  to  permit  of  the 
observation  of  the  direction  of  the  major  axis  of  its  orbit 
with  accuracy  and  the  same  is  true,  but  in  lesser  degree,  for 
the  earth.  The  values  of  the  motions  were  calculated  but 
the  precision  of  the  results  for  Venus  and  Earth  were  infe- 
rior to  those  of  Mercury  and  Mars.  This  latter  planet  with 
its  greater  eccentricity  of  orbit  shows  an  unexplained  dis- 
crepancy of  8".  i  in  the  motion  of  its  perihelion. 

According  to  Einstein's  law  of  gravitation,  the  planetary 
orbits  not  being  closed  ellipses  but  spirals,  the  path  is  slightly 
advanced  at  each  revolution  in  the  direction  of  the  motion  of 
the  planet.  The  prediction  of  Einstein  gives  a  simple  for- 
mula, namely,  that  the  orbit  of  any  planet  will  advance 
through  a  fraction  of  a  revolution  measured  by  3  IT  /  c", 
where  v  is  the  velocity  of  the  planet  and  c  the  velocity  of 
light.  The  values  of  v  for  each  of  the  planets  is  known  with 
great  exactness.  For  the  four  inner  planets,  the  Einstein 
motions  per  century  are:  42 ".9  for  Mercury,  8". 6  for  Venus, 
3".8  for  the  Earth  and  i".3  for  Mars;  showing  a  remarkable 
agreement  for  Mercury  but  little  improvement  in  the  dis- 
crepancy of  Mars.  Further  disagreements  between  observa- 
tion and  motion  calculated  on  the  Newtonian  theory  were 
found  by  Newcomb,  the  most  important  being  io".2  in  the 
node  of  Venus  and  o".88  in  the  eccentricity  of  Mercury. 
The  Einstein  theory  does  not  apply  to  the  two  last  discord- 
ances. Attention  should  thus  be  called  to  the  fact  that  the 
motion  of  the  perihelion  of  Mercury  is  not  the  only  wide  di- 
vergence from  the  law  of  gravitation  found  in  the  discus- 
sion of  the  motions  of  the  planets.  Its  value  is  the  largest, 


EINSTEIN  THEORY  VERIFIED?  453 

it  is  true,  but  the  other  disagreements  are  none  the  less  im- 
portant in  spite  of  their  smaller  size. 

Before  we  can  accept  the  Einstein  theory  as  the  true  ex- 
planation of  the  motion  of  the  perihelion  of  Mercury,  we 
must  first  exhaust  all  other  explanations  arising  from  the 
Newtonian  law  of  gravitation,  and  all  possible  deflections  in 
the  photographs  caused  by  refraction,  etc.  If  the  unex- 
plained motion  of  the  perihelion  had  been  Leverrier's  38" 
instead  of  Newcomb's  42",  the  Einstein  prediction  of  43" 
would  hardly  have  been  received  with  the  universal  acclaim 
accorded  the  almost  perfect  agreement.  Newcomb,  many 
years  ago,  showed  that  the  motion  of  Mercury  can  be  com- 
pletely accounted  for  under  several  hypotheses,  but  unfortu- 
nately in  explaining  away  the  discordance  in  Mercury's  mo- 
tion, discordances  were  introduced  into  the  other  planets  so 
that  the  last  state  appeared  no  better  than  the  first.  There 
are  three  possibilities: 

1 i )  A  non-spherical  sun 

( 2 )  A  ring  of  matter  surrounding  the  sun 

( 3  )  The  number  2  in  Newton's  law  is  only  a  first  approxi- 
mation. 

According  to  Charles  Lane  Poor,1  "  the  ordinary  classical 
methods  of  physical  and  astronomical  research  can  fully  ex- 
plain all  of  the  observed  phenomena;  the  motions  of  the 
planets  can  be  fully  accounted  for  by  the  presence  of  matter 
known  to  exist,  and  the  light  deflection,  if  real,  can  be  ex- 
plained as  the  result  of  refraction  through  the  cosmic  dust 
surrounding  the  sun." 

It  was  shown  by  Poor  that  if  the  equatorial  diameter  of 
the  sun  exceeds  the  polar  diameter  by  an  angle  of  o".io,  it 
will  cause  a  motion  of  the  perihelion  of  Mercury  amounting 
to  but  7"  per  century.  Although  from  theoretical  grounds 
the  rotation  of  the  sun  should  cause  a  bulging  of  the  sun  at 
the  equator,  such  an  effect  has  never  been  detected  with 
certainty  (see  Chapter  VII).  The  excess  of  equatorial  di- 
ameter of  the  sun  can  hardly  amount  to  more  than  o".o5, 
and  this  would  affect  the  orbit  of  Mercury  but  little. 

1  Gravitation  versus  Relativity,  227,  1922. 


454  ECLIPSES  OF  THE  SUN 

It  was  next  shown  by  Poor  that  the  assumption  of  a  lens- 
shaped  material  appendage  surrounding  the  sun  will  be 
sufficient  to  explain  not  only  the  motion  of  Mercury,  but 
practically  all  of  the  other  divergences  from  the  law  of  gravi- 
tation as  well.  The  method  adopted  by  him  was  one  very 
familiar  in  practice,  namely,  that  of  introducing  as  many 
constants  into  the  calculation  as  there  are  quantities  to  be 
allowed  for.  In  the  solar  corona  there  is  evidence  of  an  ap- 
pendage to  the  sun  and,  furthermore,  the  zodiacal  light  gives 
an  indication  of  an  envelope  stretching  out  from  the  sun 
vastly  farther  than  the  corona.  The  corona  is  more  or  less 
symmetrical  with  respect  to  the  sun's  equator,  while  the  plane 
of  the  zodiacal  light  is  that  of  the  earth's  orbit.  For  the 
purpose  of  simplicity  of  computation,  it  was  necessary  to 
make  several  more  or  less  arbitrary  hypotheses. 

According  to  Poor  (loc.  cit.,  p.  237),  "  there  is  apparently 
no  mechanical  nor  physical  reason  for  the  non-existence  of 
a  group,  or  groups,  or  bodies,  sufficient  to  explain  all  the 
irregularities  in  the  motions  of  the  planets.  Thus,  all  the 
discordances,  including  that  of  the  perihelion  of  Mercury, 
can  readily  be  accounted  for  by  the  action,  under  the  New- 
tonian law,  of  matter  known  to  be  in  the  immediate  vicinity 
of  the  sun  and  planets."  Such  a  distribution  of  material 
would  cause  a  true  atmosphere  surrounding  the  sun  which 
would  not  be  of  the  uniform  density  (assumed  for  ease  in 
computation),  but  which  would  be  very  dense  close  to  the 
sun's  surface. 

An  atmosphere  of  this  character  would  cause  refraction 
on  a  beam  of  light  when  it  passed  through  this  atmosphere. 
In  fact,  Poor  assumes  that  such  a  refraction  is  a  sufficient 
cause  to  explain  the  whole  of  the  bending  of  the  rays  of  light 
at  the  1919  eclipse.  On  the  other  hand,  Eddington  *  shows 
that  "  at  a  height  of  400,000  miles  above  the  surface  of  the 
sun  the  refractive  index  required  is  1.0000021.  This  cor- 
responds to  air  at  ^  atmosphere,  hydrogen  at  -7V  atmos- 
phere, helium  at  -*V  atmospheric  pressure.  It  seems  obvious 
that  there  can  be  no  material  of  this  order  of  density  at  such 
a  distance  from  the  sun.  If  there  were,  the  pressure  on  the 

1  Space,  Time  and  Gravitation,  p.  121. 


EINSTEIN  THEORY  VERIFIED?  455 

sun's  surface  of  the  columns  of  material  involved  would  be  of 
the  order  of  10,000  atmospheres;  and  we  know  from  spec- 
troscopic  evidence  that  there  is  no  pressure  of  this  order." 

Dr.  Harold  Jeffreys  L  discussed  the  effect,  on  the  peri- 
helion of  Mercury  and  on  the  shifting  of  star  places,  of 
matter  known  to  be  in  the  immediate  vicinity  of  the  sun  and 
planets,  namely,  the  solar  corona  and  the  zodiacal  light,  in 
the  following  manner:  "  You  can  determine  the  density  per- 
fectly from  the  luminosity.  You  know  the  intensity  of  light 
that  falls  on  these  masses;  you  know  the  distance;  you  have 
a  fair  idea  of  the  physical  constitution ;  and  you  need  simply 
compare  these  with  the  light  that  you  receive  from  the  sky 
at  midday  to  get  a  direct  estimate  of  the  amount  of  these 
two  deflections  that  can  be  got  from  this  mass  that  is  known 
to  exist.  The  results  are  rather  startling.  There  is  enough 
matter  in  the  zodiacal  light  to  account  for  one  three-thou- 
sandth of  the  motion  of  the  perihelion  of  Mercury.  There 
is  enough  matter  in  the  corona  to  account  for  about  10  8  of 
the  motion  of  the  perihelion  of  Mercury.  The  solar  corona, 
by  means  of  its  refraction,  is  capable  of  producing  one- 
millionth  of  the  observed  deflection  of  the  star  images.  So 
I  think  it  is  clear  that  the  only  reasonable  interpretation  one 
can  make  of  these  two  shifts  is  that  they  are  due  to  some 
property  of  the  law  of  gravitation." 

Another  method  of  explaining  the  motion  of  Mercury  un- 
der Newton's  law  was  that  suggested  many  years  ago  by 
Hall.  If  the  exponent  2  in  the  inverse  square  law  is  in- 
creased until  it  becomes  2.00000016,  then  the  effect  on  the 
perihelia  of  the  inner  planets  as  computed  by  Newcomb  be- 
comes: Mercury  43". 3 7,  Venus  1 6". 98,  Earth  io".45  and 
Mars  5^.55.  While  the  values  of  Mercury  and  Mars  agree 
well  with  the  observed  displacements,  those  of  Venus  and  the 
Earth  are  very  wide  of  the  true  values. 

Of  the  utmost  importance  to  the  Einstein  theory  is  the  ac- 
curate determination  of  the  shift  of  the  spectral  lines  in  the 
sun  towards  the  red  when  the  wave-lengths  are  compared 
with  those  derived  under  terrestrial  conditions.  This  spec- 
troscopic  test  is  not  based  on  mathematical  reasoning  of  such 

1  Monthly  Notices,  R.  A.  S.f  80,  n6,  138,  1919. 


456  ECLIPSES  OF  THE  SUN 

difficulty  that  few  can  follow  the  methods,  but  simply  and 
solely  on  the  theory  of  relativity  that  motions  are  relative. 
According  to  the  principle  of  equivalence,  Einstein  asserts 
that  a  ray  of  light  falling  from  the  ceiling  to  the  floor  of  a 
room  moves  exactly  the  same,  no  matter  whether  it  is  as- 
sumed that  the  light  moves  in  the  earth's  gravitational  field 
or  with  respect  to  a  system  of  axes  of  reference  moving  at 
the  rate  of  32  feet  per  second  upwards.  The  result  of  light 
moving  in  a  gravitational  field  cannot  be  distinguished  from 
a  Doppler  effect  of  motion  in  the  line  of  sight.  Consequently, 
it  is  easy  to  calculate  the  amount  of  the  shift  caused  by  the 
gravitational  field  in  the  sun,  and  this  is  equivalent  to  a  Dop- 
pler effect  of  0.634  km  per  second.  If  the  relativity  shift  is 
verified  in  the  sun,  this  same  principle  becomes  of  the  great- 
est importance  in  the  investigations  of  other  suns,  the  more 
distant  stars.  If  we  know  the  mass  and  density  of  a  star, 
from  which  can  be  found  the  gravitational  attraction,  then 
the  star  may  be  submitted  to  the  Einstein  test.  As  will  be 
shown  later,  this  test  has  been  carried  out  with  great  success 
on  the  feeble  companion  of  the  brilliant  Sirius. 

At  first  sight  the  problem  seems  to  admit  of  a  ready  solu- 
tion. All  that  appears  necessary  is  that  the  wave-lengths  of 
the  arc  spectrum  be  determined  in  the  laboratory  with  the 
highest  degree  of  accuracy  attainable  and  that  these  wave- 
lengths be  compared  directly  with  solar  values.  Let  us  look 
back  and  see  what  the  earlier  chapters  have  shown  concern- 
ing the  history  of  spectroscopy. 

As  early  as  1890,  in  the  process  of  deriving  his  famous 
table  of  solar  wave-lengths,  Rowland  had  noticed  that  com- 
parisons of  the  solar  spectrum  with  that  of  the  electric  arc 
showed  that  the  solar  lines  were  nearly  always  shifted  to  the 
red.  It  was  thought  that  at  the  time  this  shift  was  probably 
accidental,  due  unquestionably  to  some  systematic  errors 
called  in  by  the  apparatus.  However,  a  further  study  of  the 
problem  by  Jewell  convinced  him  that  the  cause  of  the  shift 
to  the  red  could  not  be  instrumental  for  the  reason  that  the 
shift  was  far  from  being  a  constant,  since  it  varied  in  amount 
with  different  chemical  elements  and  even  different  amounts 
of  shift  appeared  for  different  lines  of  the  same  element. 


EINSTEIN  THEORY  VERIFIED?  457 

Not  being  instrumental,  the  cause  of  the  displacements 
seemed  to  be  due  to  the  fact  that  the  temperatures  and 
pressures  of  the  vapors  in  the  solar  atmosphere  were  vastly 
different  from  those  found  in  the  vapors  rendered  luminous 
by  the  electric  arc.  It  seemed  impossible  for  temperature 
to  have  any  influence  whatever  on  the  wave-lengths  either 
in  th  arc  or  in  the  sun,  and  hence  the  cause  unquestionably 
was  that  of  pressure.  Accordingly,  extensive  series  of  in- 
vestigations were  carried  out  by  Jewell,  Humphreys  and 
Mohler  1  who  came  to  the  conclusion  that  the  pressure  in  the 
sun's  reversing  layer  varied  somewhere  between  two  and 
seven  terrestrial  atmospheres  with  the  mean  value  of  five  to 
six  atmospheres.  The  surprising  feature  of  the  investigation 
was  that  the  pressures  derived  were  so  small,  being  in  fact 
much  less  than  one  might  have  supposed  to  exist  in  an  enor- 
mous body  like  the  sun. 

In  1909,  Fabry  and  Buisson  2  applied  the  interferometer 
to  the  problem  and  succeeded  in  deriving  wave-lengths  with 
a  greater  accuracy  than  had  been  possible  with  a  concave 
grating  alone.  In  investigating  the  spectrum  of  iron  in  the 
electric  arc,  it  was  found  that  as  a  whole  there  was  a  shift 
to  the  red  in  the  sun  compared  with  wave-lengths  of  iron  but 
that  the  amounts  of  the  shifts  varied  greatly,  some  of  the 
lines  even  showing  a  shift  to  the  violet.  In  comparing  di- 
rectly the  wave-lengths  of  the  solar  rays  and  the  correspond- 
ing rays  of  the  iron  arc  in  a  vacuum,  Fabry  and  Buisson 
found  that  the  solar  rays  always  showed  this  shift  towards  the 
red,  which  must  be  the  case  if  the  displacement  is  the  effect 
of  pressure  in  the  sun's  reversing  layer,  provided  the  pres- 
sure in  the  sun  is  greater  than  that  found  at  the  earth's  sur- 
face at  sea-level.  The  explanation  seemed  satisfactory  and 
sufficient,  and  as  a  result  of  a  long  series  of  observations 
very  carefully  executed  the  conclusion  reached  was  that  the 
solar  pressures  were  approximately  five  atmospheres,  thus 
being  in  good  agreement  with  the  values  of  Jewell,  Hum- 
phreys and  Mohler.  The  shift  of  the  solar  lines  to  the  red 
amounted  to  0.008  A  at  wave-length  4500  A. 

1  Astrophysical  Journal,  3,  114,  1896. 
-  Journal  de  Physique,  (4),  g,  298,  1910. 


458  ECLIPSES  OF  THE  SUN 

In  1911,  Einstein  l  called  attention  to  the  amount  of  this 
shift  and  showed  that  if  it  could  be  assumed  that  the  pres- 
sure of  the  reversing  layer  was  less  than  one-tenth  of  a  ter- 
restrial atmosphere,  then  the  displacement  of  0.008  A  was 
readily  explainable  on  the  theory  of  relativity  just  then  be- 
ing developed  by  him.  Unfortunately,  all  of  the  observa- 
tions pointed  to  much  higher  pressures  at  the  photosphere 
and  to  add  to  the  complications  Halm2  had  found  (1907) 
that  the  wave-lengths  from  the  edge  of  the  sun  were  dis- 
placed towards  the  red  with  respect  to  those  from  the  center 
of  the  sun.  Neither  pressure  nor  Einstein  shift  could  explain 
these  difficulties  nor  was  the  theory  of  anomalous  dispersion 
more  successful  when  brought  forward  by  Julius/ 

There  seemed  nothing  to  do  but  to  secure  more  observa- 
tions and  with  greater  precision.  These  observations  came 
principally  from  three  observatories,  from  Kodaikanal,  from 
Allegheny  and  from  the  Mt.  Wilson  Observatory. 

All  observers  were  now  agreed  in  finding  an  unmistakable 
shift  to  the  red  of  wave-lengths  in  the  sun  when  compared 
with  terrestrial  standards.  By  the  year  1912,  four  different 
explanations  were  offered:  (i)  pressure,  (2  )  anomalous  dis- 
persion, (3)  relativity  shift  and  (4)  a  Doppler  effect  of 
motion  of  the  solar  vapors,  advanced  by  Evershed.  Which 
of  the  four  would  survive  the  test  of  precise  observations? 

At  Mt.  Wilson  an  extended  series  of  researches  were 
undertaken  with  the  electric  furnace  wherein  both  the  tem- 
perature and  pressure  could  be  controlled  at  will.  These 
researches,  continued  up  to  the  present,  have  added  enor- 
mously to  our  knowledge  of  spectra.  In  the  investigation  of 
the  spectrum  of  iron,  Gale  and  Adams  4  found  it  possible 
to  divide  the  iron-lines  into  four  groups,  designated  by  the 
symbols  a,  b,  c,  and  d.  In  these  four  groups  the  amounts  of 
the  displacements,  for  the  same  value  of  the  wave-length 
and  same  increase  in  pressure,  are  proportional  to  the  num- 
bers i  :  1.5  :  3.4  :  6.6.  In  passing  from  the  arc  in  a  vacuum 
to  the  arc  at  ordinary  atmospheric  pressure  these  investiga- 

1  Annalen  de  Physik,  (4),  35,  905,  1911. 

2  Astronomische  Nachrichten,  173,  273,  1907. 

3  Rev.  Gen.  des  Sciences,  15,  480,  1904. 

4  Astrophysical  Journal,  35,  10,  1912. 


EINSTEIN  THEORY  VERIFIED?  459 

tors  confirmed  the  conclusions  already  advanced  by  Fabry 
and  Buisson  regarding  the  pressures  in  the  photosphere. 

In  1913,  still  another  difficulty  appeared.  Wave-lengths 
secured  with  different  strengths  of  current  and  with  the  poles 
of  the  electric  arc  placed  at  different  distances  apart  showed 
wide  variations.  Hence  there  arose  what  is  now  known  to 
spectroscopists  the  world  over  as  the  "  pole-effect/'  to  cor- 
rect which  was  devised  the  "  Pfund  arc."  The  pole-effect  at 
times  may  be  quite  large.  For  the  sodium  pair  at  6154  A  and 
6160  A,  for  instance,  the  displacement  may  amount  to  as 
much  as  0.52  A.  The  Stark-effect  and  the  Zeeman-effect 
added  still  further  to  the  confusion.  The  problem  had  thus 
become  a  frightfully  complicated  one. 

Still  another  cause  of  discord  appeared  in  the  year  1915 
when  it  was  recognized  by  all  observers  that  the  amounts 
of  shifts  varied  with  the  intensities  of  the  solar  lines.  Not 
only  were  the  wave-lengths  at  the  edge  of  the  sun  shifted 
towards  the  red  with  varying  amounts  depending  on  in- 
tensity when  compared  with  lines  from  the  center  of  the  sun. 
but  also  the  lines  from  the  center  of  the  sun  were  likewise 
shifted  towards  the  red  compared  with  the  corresponding 
lines  of  the  electric  arc,  but  these  displacements  varied  in 
an  entirely  different  manner  with  change  in  intensity. 

Theoretically,  the  amounts  of  the  pressure  shifts  should 
vary  directly  as  the  cube  of  the  wave-lengths.  The  observed 
displacements  certainly  did  not  exhibit  any  verification  of 
this  law,  and  hence  of  necessity  it  was  recognized  either  that 
the  pressures  at  the  solar  surface  were  very  small  or  else 
their  effects  were  masked  by  other  causes.  But  what  were 
the  other  causes?  A  close  examination  of  the  observed 
quantities  brought  to  light  the  fact  that  the  displacements 
from  the  same  line  of  the  spectrum  gave  very  different  re- 
sults on  different  days  of  observation,  and  moreover  even 
the  lines  of  the  same  group  with  same  intensities  and  same 
wave-lengths  gave  discordant  values.  It  was  evidently  not 
sufficient  for  the  purpose  to  be  satisfied  with  having  a  stable 
form  of  apparatus.  The  temperature  must  be  kept  rigor- 
ously constant  during  the  observations,  the  grating  must  be 
covered  in  precisely  the  same  manner  by  the  rays  of  light 


460  ECLIPSES  OF  THE  SUN 

in  each  exposure,  but  most  important  of  all,  it  was  no  longer 
sufficient  to  expose  first  to  the  sun  and  then  to  the  electric 
arc,  but  the  exposures  had  to  go  on  strictly  simultaneously. 
These  refinements  when  carried  out  imposed  on  the  observa- 
tions a  much  more  satisfactory  internal  agreement  than  had 
hitherto  been  found. 

But  what  was  the  cause  of  the  systematic  deflections  to- 
wards the  red?  Apparently,  pressure  was  not  the  chief 
cause.  The  shifts  towards  the  red  throughout  all  wave- 
lengths seemed  to  behave  exactly  like  a  Doppler  effect  of 
motion  in  the  line  of  sight.  It  therefore  seemed  possible 
that  these  motions  were  actually  caused  by  the  vapors  de- 
scending towards  the  surface  of  the  sun  or  else  withdrawing 
away  from  the  earth.  When  comparing  the  center  of  the 
sun  and  the  lines  of  the  electric  arc,  the  dependence  of  the 
amounts  of  the  shifts  on  the  intensities  of  the  lines  could  be 
readily  explained  by  making  plausible  assumptions  regard- 
ing the  heights  of  the  various  layers  in  which  the  solar  lines 
originate,  but  it  was  difficult  to  apply  this  same  explanation 
to  the  shifts  of  the  lines  at  the  edge  of  the  sun  when  com- 
pared with  those  in  the  center.  Evershed  and  Royds  l  were 
therefore  forced  to  the  bizarre  conclusion  that  the  earth  was 
the  cause  of  the  displacements  and  that  this  small  planet  of 
ours  exerted  a  repulsion  on  vapors  in  the  sun's  atmosphere. 
The  authors  themselves  recognized  the  absurdity  of  the 
earth-repulsion  theory  for  there  seemed  no  reason  why  the 
other  planets  should  not  give  evidence  of  similar  repulsions. 
Evershed 2  therefore  put  this  theory  to  a  crucial  test  to  dis- 
cover whether  there  was  a  similar  effect  from  the  earth's 
twin-sister,  Venus.  For  this  purpose,  a  comparison  was  made 
between  the  spectrum  of  Venus  and  the  spectrum  of  day- 
light, that  is  to  say,  the  whole  of  the  sun's  hemisphere  turned 
towards  the  earth.  The  observations  were  carried  out  in 
1918  while  the  angle  from  Venus  to  sun  to  earth  changed 
from  45°  to  135°.  These  observations  seemed  to  give  satis- 
factory proof  to  Evershed  of  a  Venus-effect,  but  these  con- 
clusions were  not  confirmed  by  the  Mt.  Wilson  observers  8 

1  Kodaikanal  Bulletin,  49,  1916.  3  As  trophy  steal  Journal,  S3,  380,  1921. 

2  Observatory,  42,  51,  1919. 


LICK  OBSERVATORY  CAMERA  FOR  TESTING  THE  EINSTEIN  EFFECT, 
AUSTRALIA,  1922 


SAILORS  OF  THE  AUSTRALIAN  NAVY  ASSISTING  THEIR  COUSINS  FROM  CANADA  IN  ERECTING 

THE  EINSTEIN  CAMERA 


EINSTEIN  THEORY  VERIFIED?  461 

who  in  1919  and  1920  made  special  observations  towards  the 
same  end. 

Meanwhile  the  whole  world  had  been  electrified  by  the  re- 
sults of  the  1919  eclipse  which  showed  stellar  deflections  con- 
firming in  such  splendid  manner  the  Einstein  theory.  It  was 
but  natural  that  many  scientists  would  claim  that  they  had 
discovered  the  Einstein  shift  in  the  sun  —  but  many  of  these 
contributions  exhibited  little  knowledge  of  the  difficulties 
underlying  the  problem.  For  lack  of  space  it  will  be  im- 
possible here  to  go  into  details. 

When  one  refers  to  the  widely  different  values  of  the 
solar  rotation  obtained  as  the  result  of  an  extended  series 
of  most  careful  and  refined  measures  carried  out  by  the 
most  skillful  of  the  world's  solar  spectroscopists,  as  given  in 
Chapter  XVIII,  one  glimpses  some  of  the  difficulties  of  the 
problem.  The  verification  of  the  Einstein  prediction  by  sun- 
arc  displacements  is  even  a  more  difficult  task  than  that  of 
measuring  the  solar  rotation  by  spectroscopic  methods. 

The  problem,  however,  became  much  simplified  in  1923 
and  1924  by  the  recognition  by  a  number  of  competent  au- 
thorities, almost  simultaneously  and  from  a  number  of  dif- 
ferent lines  of  attack,  that  the  pressures  of  the  photosphere 
were  much  less  than  previously  had  been  supposed.  It  had 
already  become  evident  that  the  relativity  shift  could  not 
be  determined  by  measuring  a  few  lines  and  making  plausible 
assumptions  regarding  pressures  and  various  other  effects. 
To  reach  a  definite  conclusion  it  was  manifestly  necessary  to 
carry  out  a  much  more  extensive  program  of  work,  both  on 
the  sun  and  in  the  laboratory.  The  goal  of  such  work  is  in- 
deed the  determination  of  a  satisfactory  system  of  arc  and 
solar  wave-lengths  each  reduced  to  the  international  stand- 
ard, and  with  the  wave-lengths  in  sun  and  arc  freed  as  much 
as  possible  from  possible  sources  of  errors.  Such  investiga- 
tions, demanding  the  most  refined  instrumental  equipment 
and  specialized  training  of  the  highest  order  that  can  come 
only  from  long  years  of  practical  research,  seem  to  have  been 
realized. 

Wave-lengths  of  the  highest  degree  of  reliability  have 
been  determined  at  the  Allegheny  Observatory  in  coopera- 


462 


ECLIPSES  OF  THE  SUN 


tion  with  the  Bureau  of  Standards.  The  method  of  securing 
the  photographs  was  by  means  of  a  powerful  grating  speo 
trograph  and  interferometer,  the  wave-lengths  being  accu- 
rate within  one  part  in  five  million  to  one  part  in  eight 
million.  These  measures  can  readily  detect  a  shift  one-tenth 
of  the  Einstein  prediction.  The  results  for  neutral  iron  are 
more  exact  than  for  any  of  the  other  elements.  Wave- 
lengths in  sun  and  vacuum  arc  must  be  carefully  reduced  to 
one  system.  From  the  differences  sun  minus  vacuum  arc  are 
then  subtracted  the  predicted  relativity  shift  2.13  x  io~°  x  X. 

SYSTEMATIC  DIFFERENCES  IN  HEIGHTS  AND  WAVE-LENGTHS* 
WITHIN  MULTIPLEX  GROUPS 


MAIN  DIAGONAL 

SIDE  DIAGONALS 

WHOLE  MULTIPLEX 

E.P. 

INTEN- 
SITY 

No 

AX 

Hgt 

No. 

AX 

Hgt. 

No 

AX 

Hgt. 

o  07  

6—40 

9 

-M  o 

1722 

ro 

+  i   4 

1^00 

19 

+  2     2 

I  tJOO 

o  93 

4-40 

15 

+  2    0 

U57 

20 

-ho  5 

7Q2 

35 

+  1     2 

Q48 

I     ~A 

2-10 

12 

+  2.0 

1400 

1  z 

—  o  i 

82^ 

25 

~ho  9 

IIOO 

o  93   • 

4-  8 

5 

+  1    2 

540 

6 

-o  6 

500 

i  r 

-fO    2 

518 

2    2-2    8 

3-10 

10 

-ho  9 

625 

17 

—  i  o 

500 

27 

-o  3 

54« 

2    4-3    2 

i-  8 

i? 

-o  9 

497 

25 

—  12 

409 

42 

—  11 

445 

2    2-2    8.  .  . 

i-  6 

27 

-2  6 

525 

34 

-2  5 

5'8 

61 

-26 

522 

3.2-4  8 

o-  6 

40 

-2  9 

406 

49 

-2  4 

36l 

89 

—  20 

3»i 

*  The  unit  for  AX  is  o  001  A. 

For  all  of  the  individual  multiplets  of  Fe,  where  there  are 
enough  spectral  lines  to  make  it  worth  while,  the  lines  can 
be  divided  into  two  groups,  depending  on  whether  the  lines 
are  on  the  main  or  on  the  side  diagonals  of  the  multiplet. 
Altogether  a  total  of  48  multiplets  involving  309  Fe  lines 
were  discussed,  the  results  in  the  table  being  separated  into 
three  series,  representing  strong,  medium  and  weak  multi- 
plets. In  each  case  are  given:  the  number  of  lines,  the  height 
in  kilometers  from  the  flash  spectrum,  and  AX  (in  units  of 
o.ooi  A),  which  is  the  equivalent  to  the  difference  sun  minus 
vacuum  arc  from  which  has  been  subtracted  the  relativity 
shift. 

It  is  evident  from  the  abundant  material  that  went  into 
the  table  that  the  value  of  AX  on  the  main  diagonals  of  the 


EINSTEIN  THEORY  VERIFIED?  463 

multiplets  averages  greater  in  size  than  the  value  on  the  side 
diagonals.  For  obvious  reasons,  the  relativity  shift  in  each 
multiplet  must  be  practically  identical  for  main  and  side 
diagonals.  Although  the  values  of  AX  for  all  of  the  lines  in 
the  table  will  average  nearly  zero,  yet  it  is  clear  that  there 
are  systematic  differences,  the  strongest  lines,  those  which 
reach  the  greatest  heights,  have  the  largest  plus  residuals, 
while  on  the  other  hand  the  weakest  lines  or  those  which 
reach  the  least  elevations,  have  the  largest  minus  residuals. 

Apparently  therefore  the  wave-lengths  in  the  sun  are 
greater  than  those  in  the  laboratory,  the  average  difference 
being  approximately  that  of  the  Einstein  relativity  shift. 

Several  attempts  have  been  made  to  explain  the  residual 
differences.  It  has  been  assumed  by  St.  John  that  the  posi- 
tive and  negative  residuals  represent  a  Doppler  shift,  and 
in  consequence  the  atoms  forming  the  strong  lines  at  great 
heights  above  the  photosphere  are  falling  toward  the  photo- 
sphere, while  on  the  contrary  the  atoms  forming  the  weak 
lines  of  less  heights  are  ascending  from  the  solar  surface.  To 
this  very  interesting  hypothesis,  Milne  takes  exception  and 
points  out  the  various  difficulties.  He  himself  advances  the 
hypothesis  of  an  asymmetrical  distribution  of  velocity  among 
the  velocities  of  agitation  of  the  individual  atoms.  Burns  1 
favors  Milne's  point  of  view.  The  present  writer  believes 
that  the  simplest  explanation  is  to  be  found  in  the  St.  John- 
Babcock  interpretation,  which  is  nothing  more  or  less  than  a 
circulation  of  Fe  vapor  in  the  chromosphere.  On  account  of 
the  higher  temperatures  and  higher  pressures  near  the  pho- 
tosphere, the  solar  activity  causes  the  Fe  atoms  to  ascend 
through  the  medium  of  thousands  of  weak  lines  of  high  ex- 
citation potential,  the  maximum  velocity  found  from  flash 
spectrum  for  Fc  being  0.2  km  per  second.  The  heights  to 
which  the  Fc  atoms  are  ejected  are  much  greater  than  can 
be  measured  by  the  lines  in  the  flash  spectrum.  In  the  upper 
reaches  of  the  chromosphere,  especially  where  the  pressures 
are  minute,  some  of  the  atoms  lose  an  external  electron  and 
become  ionized.  From  a  law  of  nature  that  "  whatever  goes 
up  must  come  down  "  the  Fe  atoms  descend  from  the  maxi- 

1  Journal  Optical  Society  of  America,  20,  212,  1930. 


464  ECLIPSES  OF  THE  SUN 

mum  heights.  In  their  descent  some  of  the  ionized  atoms  gain 
an  external  electron  and  again  become  neutral.  At  less 
heights  the  atoms  descend  through  the  medium  of  compara- 
tively few  lines  of  great  strength  but  of  low  value  of  the  ex- 
citation potential.  For  the  Fe  lines  of  highest  level  in 
the  flash  spectrum  the  velocity  is  04  km  per  second,  while 
for  the  H  and  K  lines  which  reach  the  greatest  heights  in 
the  chromosphere,  Evershed '  finds  a  residual  value  of  AX 
equal  to  0.0070  A,  equivalent  to  a  velocity  of  -f-  0.5  km  per 
second. 

From  information  given  in  earlier  chapters,  it  is  possible 
to  make  a  calculation  of  the  number  of  atoms  crossing  a  given 
level  per  second  of  time  in  their  ascents  and  descents.  The 
numbers  appear  to  be  roughly  equal. 

Consequently,  the  greater  wave-lengths  at  the  center  of 
the  sun  when  compared  with  the  arc  appear  to  be  satisfac- 
torily explained.  But  with  the  observed  shifts  to  the  red 
at  the  sun's  edge  "  it  is  another  story."  Motions  of  circula- 
tion assumed  in  the  sun  must  be  directed  toward  or  away 
from  the  center  of  the  sun.  At  the  sun's  limb  these  motions 
are  at  right  angles  to  the  line  of  sight  to  the  terrestrial  ob- 
server, and  hence  they  cannot  cause  a  Doppler  effect  of 
motion  in  the  line  of  sight.  As  a  consequence,  the  spectrum 
of  the  limb  of  the  sun  compared  with  the  iron  arc  in  vacuum 
should  exhibit  a  constant  deflection,  independent  of  the  in- 
tensities of  the  solar  lines  under  consideration,  and  this  con- 
stant value  should  be  that  demanded  by  the  relativity 
prediction;  which,  however,  observations  do  not  confirm. 
Hence  St.  John  is  forced  to  assume  a  "  limb  effect  "  caused  by 
molecular  scattering  as  a  result  of  the  great  differences  in  the 
lengths  of  paths  traversed  by  rays  coming  tangentially  from 
the  edge  of  the  sun  compared  with  those  from  the  sun's  center. 

This  explanation  of  the  limb  effect  seems  to  be  a  make- 
shift and  no  great  confidence  can  be  felt  in  it  until  verified 
by  more  complete  observations.  Unfortunately,  the  neces- 
sity of  assuming  such  an  effect  enormously  weakens  all  of 
the  arguments  tending  to  prove  that  the  Einstein  prediction 
has  been  verified  to  exist  in  the  solar  spectrum.  Unques- 

1  Monthly  Notices,  R.  A.  S.,  90,  186,  1929. 


EINSTEIN  THEORY  VERIFIED?  465 

tionably  the  wave-lengths  in  the  sun  are  greater  than  those 
in  the  terrestrial  laboratory,  unquestionably  the  chief  cause 
of  the  greater  wave-lengths  is  the  slowing  up  of  the  atomic 
clock  in  the  sun  —  but  there  are  minor  differences  in  wave- 
length for  which  there  is  now  no  adequate  explanation.  For- 
tunately, Adams  *  at  Mt.  Wilson,  from  measures  of  the  spectra 
of  the  companion  of  Sirius,  in  a  spectacular  manner  has  not 
only  confirmed  the  validity  of  the  theory  of  generalized 
relativity,  but  at  the  same  time  has  shown  that  densities 
50,000  times  the  density  of  water  can  exist  in  the  atmos- 
pheres of  dwarf  stars  of  early-type  spectra. 

Since  the  first  burst  of  enthusiasm  in  1919,  following  the 
measures  made  by  the  British  astronomers  at  the  total  eclipse 
of  that  year,  scientists  have  had  a  chance  to  examine  more 
fully  into  the  theory  of  the  bending  of  the  light  of  a  star 
when  the  stellar  beam  passes  close  to  the  sun.  The  prob- 
lem is  after  all  not  a  novel  one  in  scientific  thought,  but  dates 
back  more  than  one  hundred  years  to  the  celebrated  English 
physicist  Cavendish  who  calculated,  on  the  basis  of  the 
corpuscular  theory,  the  amount  that  the  corpuscles  of  light 
are  bent  towards  the  sun  in  passing  it.  As  long  ago  as  1801, 
Dr.  J.  von  Soldner  derived  formulas,  which  were  published  in 
Bode's  Astron.  Jahrbuch  for  1804,  from  which  the  deflection 
of  light  at  the  edge  of  the  sun  on  the  corpuscular  theory  was 
computed  to  be  o."84.  (Using  modern  values  of  the  as- 
tronomical constants,  this  value  is  changed  to  o/'Sy.)  This 
deflection,  based  on  an  erroneous  theory  of  light,  is  the  so- 
called  Newtonian  deflection.  The  mathematical  discussion 
should  not  under  any  circumstances  be  confused  with  the 
prediction  by  Einstein  resulting  from  his  splendid  develop- 
ment of  the  general  theory  of  relativity. 

It  should  be  clearly  borne  in  mind,  however,  that  in  the 
progress  of  the  mathematical  treatment  by  Einstein  the 
equations  became  very  complicated;  and  for  the  sake  of  sim- 
plicity, of  securing  a  set  of  equations  that  could  be  manipu- 
lated, it  was  necessary  to  make  certain  assumptions.  The 
theory  coming  from  his  hands  is  therefore  not  in  its  most 
generalized  form,  and  it  is  true  only  in  so  far  as  the  restric- 

1  Proc.  Nat.  Acad.  Sciences,  11,  382;  Observatory,  48,  337,  1925. 


466  ECLIPSES  OF  THE  SUN 

tions  imposed  by  the  selection  of  the  set  of  hypotheses  can 
represent  the  observed  facts.  These  limitations  curtail  the 
applicability  of  the  theory  —  but  the  method,  rendered  nec- 
essary for  the  purpose  of  securing  results,  is  almost  as  old  as 
science  itself.  Since  knowledge  can  be  obtained  only  as  the 
result  of  observations,  it  is  evident  that  the  velocity  of  light 
is  the  maximum  velocity  that  can  enter  into  any  observation. 

Two  great  difficulties  present  themselves  to  the  average 
mind  when  attempting  to  grasp  the  meaning  of  the  Einstein 
theory.  The  first  of  these  two  is  that  it  is  almost  impossible 
to  conceive  that  the  coordinate  time  differs  in  no  essential 
particulars  from  the  three  dimensions  of  length,  breadth 
and  thickness.  The  upholders  of  the  relativity  theory 
assert  that  the  world  of  events  is  rendered  the  simpler  by 
this  consideration  of  a  space-time  concept  of  four  dimen- 
sions, even  though  it  is  necessary,  as  a  consequence,  to  regard 
the  whole  volume  of  space  as  finite,  but  nevertheless  without 
a  boundary.  As  a  matter  of  fact,  no  one  can  possibly  picture 
the  curvature  of  a  four-dimensional  continuum  for  the  rea- 
son that  there  is  nothing  at  all  in  our  experience  comparable 
with  it. 

There  are  some  individuals  with  mathematically  trained 
minds  who  have  persuaded  themselves  that  they  can  really 
grasp  the  conception  of  the  four-dimensional  world  of  space- 
time;  but  most  of  these  rebel  and  refuse  to  be  convinced 
when,  as  a  second  consequence  of  the  Einstein  theory,  it  is 
necessary  to  assume  that  gravitation  should  receive  no  treat- 
ment different  from  that  accorded  the  other  "  forces  "  of 
nature.  Gravitation  is  therefore  explained  by  Einstein  as 
being  due  to  the  curvature  of  space  and  time.  Certain  mas- 
ter minds,  like  Eddington,1  state  that  a  great  simplification 
has  taken  place;  and  that  "  we  have  realized  for  the  first 
time  that  a  world  without  gravitation  (without  curvature) 
would  be  more  specialized  and  stand  more  in  need  of  ex- 
planation than  the  actual  world  disturbed  by  gravitation." 

Eddington's  colleague  at  Cambridge,  Sir  Joseph  Larmor,1' 
finds  difficulty  in  believing  that  "  the  world  of  so-called  clas- 
sical physical  science  is  an  illusion,  an  imperfect  picture  of  a 

1  Sdentia,  33,  313,  1923.  '2  Phil.  Mag.  45,  243,  1923. 


EINSTEIN  THEORY  VERIFIED?  467 

reality  depending  symbolically  on  a  field  of  fourfold  quasi- 
geometrical  algebra,  of  a  presumed  cosmos  deep  below  the 
sensual  sources  of  our  concrete  physical  knowledge,  of  which 
we  may  obtain  shadowy  glimpses,  but  which  is  as  yet  quite 
beyond  any  direct  grasp  or  intuition/'  Furthermore,  in  this 
important  paper  (loc.  cit.}  Larmor  states  the  belief  that  the 
principle  of  equivalence  between  a  field  of  gravitation  and 
a  field  of  acceleration  seems  definitely  to  have  failed.  Lar- 
mor makes  a  careful  study  of  the  whole  problem  and  comes 
to  the  definite  conclusion  (derived  before  the  date  of  the 
1922  eclipse)  that  the  maximum  deflection  experienced  by 
the  stars  surrounding  the  sun  at  an  eclipse  should  amount  to 
one-half  that  predicted  by  the  Einstein  theory,  or  in  other 
words,  o".87  at  the  limb  of  the  sun. 

The  eclipse  of  1919  certainly  did  not  furnish  a  decisive 
answer  regarding  the  verification  of  the  Einstein  problem. 
The  work  at  best  was  pioneer  in  its  nature,  but  it  did  give 
invaluable  information  regarding  the  best  method  of  pro- 
cedure for  researches  of  the  future.  At  the  eclipse  of  1922, 
four  improvements  over  the  methods  adopted  in  1919  were 
incorporated.  First,  the  coelostat  mirrors  were  eliminated, 
the  cameras  being  pointed  directly  at  the  sun.  Second,  the 
lenses  employed  were  quadruple,  and  not  the  ordinary  achro- 
matic combination  of  flint  and  crown  glass  lenses;  thus  en- 
suring a  flat  field  over  a  very  large  area.  Third,  instead  of 
trusting  to  luck  that  the  driving  mechanism  would  perform 
its  duties  perfectly,  auxiliary  guiding  on  a  star  during  the 
eclipse  exposures  was  attempted.  Fourth,  the  securing  of 
check  plates  was  thoroughly  accomplished. 

The  observers  devoting  themselves  mainly  to  the  Einstein 
problem  were  the  British  party  of  Jones  and  Melotte  who 
located  on  Christmas  Island  in  the  Indian  Ocean,  and  a  com- 
bined German-Dutch  expedition  with  headquarters  also  on 
Christmas  Island.  In  Northwest  Australia  was  the  Lick- 
Crocker  expedition  under  the  direction  of  Dr.  W.  W.  Camp- 
bell, a  Canadian  party  from  the  University  of  Toronto  con- 
sisting of  Dr.  C.  A.  Chant  and  Dr.  R.  K.  Young,  and  Mr. 
J.  Evershed  from  the  Kodiakanal  Observatory  of  India. 

The  British  observers  at  Christmas  Island  spent  six  months 


468  ECLIPSES  OF  THE  SUN 

at  the  eclipse  site  in  order  to  secure  the  check  plates  in  ad- 
vance of  the  eclipse  and  also  that  photographs  might  be  se- 
cured of  the  southern  skies  to  be  used  for  measuring  the  mag- 
nitudes of  the  southern  stars.  The  observers  unfortunately 
found  conditions  nearly  identical  with  those  experienced  in 
Sumatra  at  the  eclipse  of  1901.  The  skies  were  so  covered 
with  thin  haze  or  heavier  clouds  almost  every  night  that 
neither  the  check  plates  nor  the  other  stellar  photographs  so 
much  desired  could  be  secured.  To  add  to  the  continued  dis- 
appointment of  the  observers,  the  sky  was  clouded  during 
the  critical  minutes  on  September  21  and  no  eclipse  photo- 
graphs were  secured  either  by  the  English  or  by  the  German- 
Dutch  parties. 

The  Einstein  equipment  of  the  Lick  Observatory  party 
consisted  of  one  pair  of  lenses  each  of  5  inches  aperture  and 
15  feet  in  focal  length,  and  another  pair  of  4  inches  aper- 
ture and  5  feet  in  focal  length.  Each  lens  was  a  quadruplet 
and  the  specifications  required  a  flat  field  giving  good  star 
images  over  plates  17x17  inches.  The  photographic  films 
were  coated  on  plate  glass  a  quarter  of  an  inch  in  thickness. 

Wallal,  the  headquarters  of  a  sheep  ranch  on  the  bleak 
shores  of  Northwest  Australia,  was  chosen  as  the  scene  of 
operations  by  the  Lick  and  Canadian  observers.  It  was 
found  not  to  be  practicable  to  send  an  observer  with  the 
Einstein  cameras  to  Wallal  three  months  or  more  before  the 
eclipse  in  order  to  mount  and  adjust  the  cameras  and  secure 
the  necessary  check  plates  in  the  night  skies,  and  conse- 
quently it  was  decided  that  Trumpler  should  secure  these 
plates  in  Tahiti  in  the  Southern  Pacific  where  the  living  con- 
ditions were  more  kindly.  The  latitude  of  Tahiti  (17°  32'$) 
differed  little  from  that  of  Wallal  (19°  45'  S)  and  it  was 
thought  that  the  temperatures  at  night  at  Tahiti  would  differ 
little  from  those  by  day  at  Wallal.  Dr.  Trumpler,  in  spite 
of  much  interference  from  clouds,  secured  the  necessary 
plates,  not  only  with  each  of  the  four  Lick  objectives  but  with 
the  two  Canadian  cameras  as  well. 

As  an  extra  precaution,  it  was  decided  to  photograph  on 
each  plate  taken  at  Tahiti  not  only  the  critical  star  group 
where  the  sun  would  be  projected  at  the  time  of  the  eclipse, 


EINSTEIN  THEORY  VERIFIED?  469 

but  also  an  auxiliary  group  having  the  same  declination  but 
with  a  right  ascension  six  hours  farther  east.  The  night  be- 
fore the  eclipse,  the  photographic  plates  to  be  used  for  the 
first  half  of  the  eclipse  set  were  exposed  to  the  auxiliary 
group,  while  the  second  half  of  the  eclipse  plates  were  ex- 
posed to  the  star  group  at  night  following  the  eclipse.  The 
use  of  the  second  star  group  thus  furnished  a  double  check 
on  the  accuracy  of  the  eclipse  photographs  and  by  their  means 
it  was  hoped  to  eliminate  completely  any  systematic  er- 
rors that  might  possibly  creep  into  the  Tahiti  plates  be- 
cause of  their  not  having  been  actually  taken  at  the  eclipse 
site. 

The  Australian  government  set  a  high  standard  of  scien- 
tific cooperation.  All  eclipse  observers  were  carried  by  rail- 
road free  of  charge  from  Sydney  to  Freemantle  and  return. 
They  were  transported  by  ship  from  Freemantle  to  Broome 
and  then  to  Wallal  and  return.  The  heavy  equipment, 
weighing  about  thirty-five  tons,  was  landed  on  the  beach 
and  transferred  a  mile  and  a  half  inland  to  the  observing 
camp.  The  labor  needed  for  this  and  for  most  of  the  opera- 
tions required  by  nearly  a  month's  stay  at  Wallal,  including 
complete  sleeping  and  dining  service,  was  rendered  entirely 
gratis.  In  the  estimation  of  Director  Campbell  *  "  the  hos- 
pitality extended,  the  interest  shown,  and  the  assistance  of- 
fered, were  of  a  standard  higher  than  I  have  ever  observed 
in  any  other  part  of  the  world  on  any  occasion." 

The  population  of  whites  in  the  Wallal  region  numbered 
only  six  persons.  Many  serious  obstacles  had  to  be  overcome 
in  the  short  time  available  before  the  instruments  were  erected 
and  accurately  adjusted.  Perhaps  the  greatest  trials  were 
the  dust  and  the  flies.  The  eclipse  site  was  in  the  bed  of  an 
extinct  lake,  and  on  account  of  the  small  annual  rainfall  the 
ground  was  very  dry  and  a  cloud  of  fine  dust  was  raised  at 
each  step  by  a  person  walking,  and  a  gentle  wind  carried  it  to 
all  portions  of  the  plate  holders  and  optical  parts  of  the  deli- 
cate apparatus.  In  order  to  attempt  to  keep  down  the  dust, 
five  blacks  were  continuously  employed  throughout  the 
whole  stay,  carrying  coarse  sand  and  distributing  it  on  the 

1  Publ.  A.  S.  P.,  35,  n,  1923. 


470  ECLIPSES  OF  THE  SUN 

ground  surrounding  the  instruments.  On  the  morning  of 
eclipse  day,  September  2 1 ,  green  boughs  were  cut  from  trees 
and  placed  on  the  ground  and  water  plentifully  sprinkled 
around  in  order  to  attempt  to  diminish  the  radiation  from 
the  heated  soil.  Fortunately  for  the  observers,  the  work  of 
adjusting  and  erecting  the  apparatus  was  not  interfered  with 
by  clouds.  Only  occasionally  during  the  month's  stay  at 
Wallal  were  clouds  seen  and  these  never  appeared  far  above 
the  horizon. 

The  account  (loc.  cit.}  by  the  director  of  the  Lick  Ob- 
servatory of  the  preparation  for  the  eclipse  gives  one  an 
inkling  of  the  reasons  why  the  Lick-Crocker  expeditions  have 
established  such  a  splendid  record  for  scientific  work  accom- 
plished at  eclipses.  The  Lick  program  is  always  carefully 
planned,  the  equipment  is  the  very  best  that  modern  engi- 
neering can  supply,  the  apparatus  is  thoroughly  tested  at 
home  so  that  when  it  arrives  at  the  eclipse  site  it  can  be  read- 
ily erected  and  adjusted.  Added  to  the  above  is  the  vast 
experience  gained  by  the  successful  observations  of  eclipses 
in  all  quarters  of  the  globe  during  the  past  thirty  years.  The 
author  wishes  to  pay  high  tribute  to  his  friend,  the  director 
emeritus  of  the  Lick  Observatory  and  also  to  the  friends  of 
the  Observatory  whose  scientific  interest  and  financial  back- 
ing have  made  the  Lick-Crocker  expeditions  possible. 

On  eclipse  day,  September  2 1 ,  in  Wallal  not  a  single  cloud 
was  visible  from  morning  to  night  —  the  astronomers  could 
not  therefore  lay  the  blame  on  the  weather  if  their  photo- 
graphs turned  out  unsuccessfully. 

The  most  important  problem  for  this  eclipse  was  unques- 
tionably the  photographs  for  testing  the  Einstein  problem. 
The  measurements  of  the  Lick  plates  by  Campbell  and 
Trumpler  (Lick  Observatory  Bulletin,  No.  346)  are  of  the 
high  precision  one  naturally  expects  in  work  emanating 
from  the  Lick  Observatory.  The  methods  adopted  were  the 
same  as  those  of  the  British  astronomers  in  1919  and  the 
Canadians  in  1922  and  consisted  essentially  in  comparing 
each  of  the  eclipse  plates  with  a  check  plate  of  the  same 
stellar  region  but  with  the  sun  absent.  The  most  important 
sources  of  error  that  could  influence  the  results  were: 


EINSTEIN  THEORY  VERIFIED?  471 

1.  Distortions  of  the  star  places  by  the  optical  parts  of 
the  apparatus. 

2.  Disturbing  effects  in  measuring  the  star  images  of  the 
eclipse  plates  on  account  of  being  superimposed  on  the  back- 
ground of  the  corona. 

3.  Systematic  distortion  in  the  film  of  the  eclipse  photo- 
graphs.    Ross  *  has  shown  that  the  blackened  part  of  the 
corona  should  dry  more  quickly  than  the  balance  of  the  plate, 
possibly  resulting  in  a  contraction  of  the  film  towards  the 
inner  corona. 

4.  Abnormal  refraction  2  in  the  earth's  atmosphere  on 
account  of  the  cooler  temperature  inside  the  shadow-cone  of 
the  moon. 

5.  Refraction  3  in  an  extended  solar  atmosphere. 

6.  A  "  yearly  refraction  "  suggested  by  Courvoisier.4 
Unfortunately,  the  stars  surrounding  the  sun  were  very 

weak  at  the  1922  eclipse,  there  being  only  two  stars  brighter 
than  the  ninth  magnitude  within  i°  of  the  sun's  center, 
while  in  1919  there  were  seven  stars  brighter  than  the  sixth 
magnitude  in  a  similar  area.  It  was  necessary  to  prolong 
the  exposures  to  secure  as  faint  stars  as  possible.  Exposures 
of  2  mins.  and  2  mins.  5  sees,  respectively,  were  given  for 
each  of  the  Einstein  cameras.  During  totality  two  plates 
were  secured  with  each  camera,  there  being  two  cameras 
of  i5-foot  and  two  of  5-foot  focus,  the  exposures  starting 
10  sees,  after  the  beginning  and  stopping  10  sees,  before  the 
end  of  totality.  The  check  plates  secured  in  the  night  skies 
in  Tahiti  were  given  exposures  of  three  minutes.  The  results 
given  below  are  those  from  the  measures  of  the  four  plates 
with  the  i5-foot  camera  (ratio  of  aperture  to  focal  length 
1:36).  Stars  of  photographic  magnitude  10.5  showed  meas- 
urable images,  the  first  time  in  the  history  of  eclipse  work 
that  such  faint  stars  have  been  photographed.  In  the  process 
of  the  work  the  positions  of  118  separate  stars  altogether 
were  measured,  while  about  50,000  settings  were  necessary  to 
accomplish  this. 

1  Astro  physical  Journal,  52,  98,  1920. 

-  A.  Anderson,  Nature,  104,  1919-20;  Poor,  Science,  57,  613,  1923. 

3  H.  F.  Newall,  Monthly  Notices,  R.  A.  S.,  80,  22,  1919. 

4  Beob,  Ergebn.  Sternw.  Berlin,  15,  1913;  A.  N.,  211,  205,  1920. 


472  ECLIPSES  OF  THE  SUN 

The  star  images  on  the  eclipse  plates  were  round  and  sym- 
metrical. The  present  writer  has  found  in  his  experience  in 
measuring  stellar  photographs  that  a  fogging  of  the  plates 
by  daylight,  moonlight,  or  other  causes,  renders  the  edges  of 
the  star  images  ill-defined  and  fuzzy  in  appearance;  conse- 
quently, one  is  not  surprised  to  learn  from  the  Lick  report 
that  the  stars  on  the  eclipse  plates  were  not  as  sharp  in  ap- 
pearance as  those  on  the  check  photographs.  The  focus, 
however,  was  of  the  very  best. 

The  details  of  the  measurements  and  reductions  cannot  be 
given  here.  It  was  necessary  to  correct  the  measures  for  the 
effects  of  proper  motion  and  annual  parallax  of  the  stars,  for 
differential  refraction  and  aberration,  and  for  the  inclination 
of  the  plates  to  the  optical  axis.  Ten  stars  of  the  eclipse  field 
and  three  of  the  auxiliary  check  field  had  proper  motions  of 
known  amount  and  of  sensible  size.  One  star  ( Beta  Virginis ) 
has  a  measured  parallax  of  o".io,  and  three  other  stars  had 
hypothetical  parallaxes  which  were  derived  from  their  known 
spectral  classes  and  proper  motions. 

Arranging  the  stars  according  to  their  distances  from  the 
sun's  center,  the  group  means  were  formed  and  are  given  in 
the  following  table: 

GROUP  MEANS  OF  OBSKRVI  D  RADIVL  DISPL \CKMI  NTS 

Mean  Dnt.         Ol»cri''d  CorraCd  Einstrin 

Group         Stars          from  Sun         rad.  dispL          rad.  displ.  Thcorv 

O  II  ft  If 

1  8  0.64  +  .64  +  .6q  -f  .70 

2  ii  1.06  +  .35  +  .46  +  .37 

3  10  1.40  +  .30  +  .39  +  .24 

4  8  1.66  -f  .16  -f  .22  +  .17 

5  Q  i. QO  +  .17  +  .21  +  .13 

6  8  2.00  -f  .15  +  .17  +  -ii 

7  II  2.22  +  .08  +  .08  +  .08 

8  T3  2.55  —  .oq  —  .14  +  .02 
Q                14               2.97                —  .04                —  .08  —  .03 

It  is  at  once  noticeable  that  the  measures  are  in  remark- 
able agreement  with  the  values  predicted  from  the  Einstein 
theory;  even  the  stars,  far  removed  from  the  sun's  center 
and  well  outside  the  region  of  the  denser  corona,  showing  the 
light-deflections  well  marked. 


.32 

62 

i".88=*=' 

'.27 

6Q 

i  ".So 

.22 

77 

1.62 

.22 

81 

1.48 

.22 

80 

1.91 

.19 

84 

1.89 

1.76 

.22 

85 

1.76 

EINSTEIN  THEORY  VERIFIED?  473 

The  Einstein  deflections  are  inversely  proportional  to  the 
angular  distance  from  the  sun's  center,  and  reducing  the 
values  for  the  separate  stars  to  the  amount  at  the  sun's  limb, 
then  there  results  for  the  four  pairs  of  plates  the  values  given 
below: 

LIGHT  DEFLECTION  AT  THE  SUN'S  LIMB 

Pair  Campbell         No.  of  Stars         Trumpler          No.  of  Stars  Plate  Mean 

i  i".72  = 

3  1-78 

4 
Mean  for  each 

observer   i".6o*=".i4  i".78=*=".n 

Mean  from  four  plates  l//.72.±"n 

Einstein  value  I/7-745 

In  addition,  the  star  field  was  divided  into  four  quadrants 
so  that  the  sun's  equator  passed  through  the  middle  of  two 
of  the  quadrants  and  the  sun's  axis  of  rotation  through  the 
other  two.  The  stars  in  each  quadrant  were  solved  sepa- 
rately by  least  squares  with  the  results  below: 

Quadrant  No.  of  Light  deflection 

Stars  at  Sun's  limb 

Preceding  sun's  equator    24  i".6i*=".2& 

Following  sun's  equator    2Q  1.68       .25 

At  sun's  north  pole    18  1.76       .26 

At  sun's  south  pole    21  1.73       .24 

Mean  of  equatorial  quadrants   ...     53  1.63       .15 

Mean  of  polar  quadrants      30  1.76       .18 

Mean  of  plates         92  1.72       .11 

These  magnificent  results  obtained  with  the  two  cameras 
of  i5-foot  focus  furnish  unmistakable  evidence  that  there  is  a 
bending  of  the  light  rays  from  the  stars  as  these  rays  pass 
close  to  the  edge  of  the  sun  and  that  the  amount  of  the 
deflection  agrees  remarkably  closely  with  that  predicted  by 
the  Einstein  theory, 

This  theory,  however,  not  only  predicts  a  total  deflection 
of  i".75  for  a  ray  of  light  just  grazing  the  sun's  surface,  but 
it  also  requires  that  the  deflections  from  the  other  rays  shall 


474  ECLIPSES  OF  THE  SUN 

vary  inversely  proportional  to  the  angular  distances  from 

the  center  of  the  sun.    It  was  for  the  purpose  of  testing  this 

distance  law  that  Campbell  obtained  photographs  with  the 

two  cameras  of  5 -foot  focus.     These  lenses  were  built  on 

lines  similar   to   those   of   1 5-foot   focus.     The   plates   of 

17x17  inches  covered  a  very  large  field  of  fifteen  degrees 

square.    Six  plates  were  secured  during  totality.    The  eclipse 

photographs  show  between  400  and  500  stars,  of  which  only 

135  to  140  of  the  best  defined  images  suitably  distributed 

over  the  plates  were  selected  for  measurement.    Although 

the  scale  is  only  one-third  of  that  secured  with  the  plates  of 

i5-foot  focus  the  observed  probable  error  is  only  1.6  times 

greater. 

The  difficulties  encountered  in  trying  to  establish  the  law 
of  distances  is  two-fold.  In  the  first  place,  the  corona  sur- 
rounding the  sun  is  so  bright  that  it  obscures  and  hence  pre- 
vents the  obtaining  of  observations  close  to  the  edge  of  the 
sun  where  the  deflections  are  large.  However,  the  chief  diffi- 
culty is  that  the  exact  scale  of  the  eclipse  plates  is  unknown 
and  can  be  obtained  only  from  the  observations  themselves. 
Since  the  scale  of  the  plate  is  directly  proportional  to  the 
distances  measured,  the  distance  law  is  superposed  on  that 
of  scale,  and  the  two  cannot  be  separated  unless  there  is  a 
large  range  of  distances  to  be  considered. 

A  masterly  discussion  l  by  Campbell  and  Trumpler  of  the 
plates  taken  by  the  5-foot  cameras  gives  the  deflection  at 
the  sun's  limb  of  i".&2±  ".15.  Combined  with  the  results 
from  the  1 5-foot  plates,  noted  above,  the  measured  deflection 
of  the  Lick  1922  photographs  turns  out  to  be  i".fj$±".og] 
in  exact  agreement  with  the  value  predicted  by  the  general- 
ized theory  of  relativity. 

In  consequence  of  the  difficulties  of  separating  the  Einstein 
deflection  from  the  scale  of  the  photographs,  noted  above, 
confirmation  at  other  eclipses  is  desirable.  At  the  eclipse  of 
1929,  the  expedition  from  Potsdam  2  employed  two  cameras 
for  measuring  the  relativity  shift.  The  results  from  four 
plates  from  the  28-foot  horizontal  camera  used  with  coelo- 

1  Lick  Observatory  Bulletin,  13,  130,  1928. 

2  Zeit.  fur  Astrophysik,  3,  171,  1931. 


EINSTEIN  THEORY  VERIFIED?  475 

stat  have  been  published.    Freundlich,  von  Kliiber  and  von 
Brunn  find  from  their  plates  a  relativity  displacement  at  the 
limb  of  the  sun  amounting  to  2". 24,  a  value  which  not  only 
differs  greatly  from  the  1922  Lick  results  but  also  contradicts 
the  value  predicted  by  Einstein.    Moreover,  in  rediscussing 
the  measures  of  the  1 5-foot  Lick  photographs,  the  Potsdam 
astronomers  find  a  value  of  2  ".05,  greatly  in  excess  of  the  de- 
flection i ".7 2  derived  by  the  Lick  observers  from  their  own 
plates.   The  larger  value  obtained  from  the  1922  plates  seems 
possibly  to  confirm  the  Potsdam  results  from  the  1929  eclipse. 

It  is  very  unfortunate  that  the  star  field  at  the  1929  eclipse 
was  ill  suited  to  attain  the  highest  accuracy,  for  the  reason 
that  of  the  total  of  18  measurable  star  images  on  the  Pots- 
dam plates,  17  of  the  stars  were  on  one  side  of  the  sun  and 
only  one  star  on  the  other  side.  This  poor  distribution 
brought  serious  complications  into  the  determination  of  the 
exact  scale  value  of  the  plates.  A  rediscussion  of  the  1929 
results  by  Jackson *  gave  1^.98  as  the  value  of  the  Einstein 
term.  A  more  thorough  discussion  by  Trumpler  derives  the 
deflection  from  the  Potsdam  plates  of  i".75  instead  of  the 
much  larger  value  of  2  ".2 4.  This  large  difference  does  not 
come  from  a  radical  change  in  methods  of  reduction,  which 
are  practically  identical,  but  rather  from  the  different  method 
of  deriving  the  scale  value  of  the  plates. 

After  reviewing  all  of  the  computations  for  the  eclipses  of 
1922  and  1929,  Trumpler  ~  comes  to  the  conclusion  that  the 
Lick  discussion  of  the  1922  plates  is  the  more  reliable,  and 
also  that  the  value  of  i"-75  for  the  1929  eclipse  is  entitled  to 
greater  confidence  than  the  larger  Potsdam  value  for  the 
reason  that  the  systematic  trends  in  the  Potsdam  residuals 
are  thereby  eliminated. 

Adopting  Trumpler 's  reductions,  the  following  table  gives 
the  results  for  the  1919,  1922  and  1929  eclipses  for  the  ob- 
served deflections  reduced  to  the  sun's  limb.  The  extreme 
distances  from  the  center  of  the  sun  at  which  stars  were 
measured  are  also  given.  The  values  are  in  excellent  agree- 
ment with  the  prediction  of  Einstein,  for  not  a  single  ob- 

1  Observatory,  $4,  292,  1931. 

2  Zeit.  fur  Astrophysik,  4,  208;  PubL  A.  S.  P.,  44,  167,  1932. 


476 


ECLIPSES  OF  THE  SUN 


servation,  except  the  plates  of  poor  definition  obtained  with 
the  astrographic  lens  at  Sobral  (p.  448),  differs  from  the 
predicted  value  by  more  than  twice  the  probable  error. 

STELLAR  DEFLECTIONS  OBSERVED  AT  THE  IQIQ  AND  1922  ECLIPSES 


Eclipse 

Expedition 

Focal  |  N°'         No.  of 
Len«thi'  Plates,       Stars 

Ang 
Dist 
from 
("enter 

Deflection 
at  Limb 

Authors 

IQIQ 
May  29 

(Sobral 
j  Sobral 
(Principe 

iQft.       7    ,                7 
ii       i    1  6    I         6-12 

ii     !    2  i           5 

i°-5 
i   5 
i   5 

i"    Q8d="    12 

o  86±      i 
i   6i±    .3 

[Dyson 
I  Eddington 
(Davidson 

fWalial 

10                2                      l8 

2    6 

*   75±    .3 

f  Young 
1  Chant 

IQ22 

Sept.  21 

|  Cordillo 
<;  Downs 

1 

5       '      2    ,              14 

2    Q 

i   77±    -3 

fDodwell 
\Davidson 

\Vallal 

15            4          ^2-85 

3  4 

1    72zh       II 

Campbell 

Wallal 

5            6      135-140 

10  4 

i   82=fc     15 

Trumpler 

Freundlich 

IQ2Q 

May  9 

Takengon 

28            4           17-18 

2    0 

i-75±    .13 

•  von  Kluber 
von  Brunn 

Neglecting  the  one  value  at  Sobral  (as  was  done  by  the 
British  astronomers),  the  weighted  mean  value  for  the  rela- 
tivity deflection  reduced  to  the  sun's  edge  is: 

1".79±".06 

No  other  effect  or  combination  of  effects  can  confirm  in 
such  an  adequate  manner  the  observed  deflections  of  eclipse 
plates  as  can  the  Einstein  prediction.  Hence  it  must  be 
concluded  that  eclipse  plates  have  abundantly  verified  the 
theory  of  relativity.  It  must  not  be  assumed,  however,  that 
thereby  the  theory  has  been  proved  to  be  true.  No  amount 
of  observational  confirmation  can  furnish  indisputable  proof 
of  a  law  of  Nature.  Even  one  exception  to  the  great  law  of 
gravitation  would  make  it  necessary  to  discard  the  law  or  to 
revise  it. 

The  theory  of  relativity  is  based  on  the  notion  that  it  is 
impossible  to  observe  absolute  motions  and  that  all  of  our 
knowledge  is  derived  from  motions  which  are  relative.  The 
theory  has  been  very  successful  in  interpreting  observations 
in  many  different  branches  of  physical  science.  Einstein 


c 
o 


w 

3 


S  a 

§  "o 

3s 

^  .£f 


EINSTEIN  CAMERA  OF  THE  KYOTO  EXPEDITION  AT  THE  1934 

ECLIPSE 


EINSTEIN  THEORY  VERIFIED?  477 

extended  the  theory  to  the  more  precise  observations  fur- 
nished by  astronomy  and  derived  a  new  law  of  gravitation. 
The  motions  predicted  by  this  new  law  differ  from  those  de- 
rived from  the  Newtonian  law  by  quantities  that  are  so  very 
minute  that  they  can  be  tested  only  as  the  result  of  very 
refined  observations.  The  Einstein  law  has  been  superior  to 
the  Newtonian  law  ( i )  in  explaining  the  motion  of  the  peri- 
helion of  Mercury,  (2)  in  interpreting  the  stellar  deflections 
observed  on  eclipse  plates,  and  (3)  in  explaining  the  ob- 
served shifts  to  the  red  of  lines  in  the  solar  spectrum  com- 
pared with  terrestrial  standards.  The  spectroscopic  prob- 
lem is  a  very  complicated  one  with  many  different  factors  to 
be  considered.  It  will  be  necessary  to  obtain  additional  ob- 
servations before  the  spectroscopic  test  is  regarded  as  entirely 
conclusive. 

The  question  now  (1935)  before  astronomers  is  whether 
we  shall  accept  the  observed  value  of  the  deflection  at  the 
sun's  limb  of  ir/-79±  ".06,  found  on  page  476,  as  a  satisfac- 
tory confirmation  of  the  value  i".75  given  by  the  relativity 
theory.  In  tabulating  these  values  the  author  has  considered 
them  worthy  of  greater  confidence  than  the  larger  displace- 
ments, between  2".o  and  2 ".2  at  the  sun's  limb,  derived  (loc. 
cit.}  by  Freundlich,  von  Kliiber  and  von  Brunn. 

The  differences  between  the  two  sets  of  reductions  show 
the  underlying  difficulty  of  the  whole  problem,  namely,  the 
practical  impossibility  of  adequately  separating  the  scale  of 
the  plates  from  the  star  deflections.  According  to  the  opinion 
expressed  by  Freundlich  and  von  Brunn,1  the  problem  de- 
mands for  the  scale  of  the  plates  the  excessively  high  accuracy 
of  no  less  than  one  part  in  one  hundred  thousand!  It  need 
hardly  be  added  that  nowhere  has  this  accuracy  been  attained 
in  the  past.  Therefore  it  would  seem  to  the  man  in  the  street 
an  evident  consequence  that  the  work  must  be  repeated  at 
future  eclipses  and  with  far  greater  accuracy  than  in  the  past. 
Such  a  recommendation  appears  obvious  —  but  unfortu- 
nately Wright "  has  found  that  no  one  of  the  star  fields  of  any 
total  eclipse  of  the  future  including  that  of  February  25, 1952, 

1  Zcit  fur  Astrophysik,  6,  218,  1933. 
-  Pitbl.  A.  S.  P.,  45>  25,  I933- 


478  ECLIPSES  OF  THE  SUN 

is  adequate  to  give  reliable  information  concerning  the  rela- 
tivity displacement. 

Measures  by  Wright  *  of  photographs  of  the  eclipse  of 
April  28,  1930,  by  the  36-inch  Lick  refractor  offer  "  no  sup- 
port for  the  suggestion  that  has  been  made  that  the  Einstein 
effect  may  be  a  consequence  of  abnormal  refraction  within  the 
cold  shadow  of  the  moon  at  the  time  of  a  total  eclipse  of  the 


sun." 


There  are  still  many  doubting  Thomases  for  whose  benefit 
it  will  be  necessary  to  accumulate  additional  observations. 
The  author  wishes  to  state  that  he  is  one  of  those  who  believe 
that  the  Einstein  theory  of  relativity  has  been  abundantly 
verified  by  astronomical  observations. 

Accepting  the  truth  of  the  relativity  theory,  de  Sitter  in 
1917  brought  forward  his  famous  prediction  that  the  veloci- 
ties of  very  distant  objects  must  show  motions  of  recession. 
At  the  time  of  his  prediction,  a  few  motions  of  distant  spirals 
were  known  as  the  result  of  observations  made  at  the  Lowell 
Observatory.  Since  then  many  velocities  have  been  accumu- 
lated at  Mt.  Wilson,  and  they  show  almost  without  exception 
that  the  spiral  nebulae  are  receding  from  us  with  velocities 
that  apparently  depend  on  the  distances,  the  greater  the  dis- 
tance, the  greater  the  velocity.  The  largest  velocity  so  far 
found  among  the  extra-galactic  nebulae  is  -f  24,500  miles 
per  second. 

The  important  point  of  de  Sitter's  theory  was  that  it  pre- 
dicted the  tendency  of  matter  to  scatter.  This  theory,  how- 
ever, was  based  on  the  assumption  that  space  is  utterly  de- 
void of  matter  —  which  is  hardly  in  harmony  with  our  ideas 
of  the  physical  universe.  Opposed  to  the  "  de  Sitter  uni- 
verse "  is  the  "  Einstein  universe."  The  theory  of  the  latter 
assumes  a  uniform  distribution  of  matter  from  which  was 
concluded  a  curvature  of  space  so  that  the  universe  appeared 
to  be  a  sphere  of  finite  radius.  In  such  a  universe  the  gravi- 
tational attraction  would  exactly  balance  the  tendency  of  the 
spiral  nebulae  to  scatter,  producing  static  conditions.  The 
fact  that  the  nebulae  are  receding  with  such  high  velocities  is 
directly  contrary  to  the  static  universe  of  Einstein. 

1  Publ.  Amer.  Astron.  Socy.,  8,  79,  1934. 


EINSTEIN  THEORY  VERIFIED?  479 

Apparently,  therefore,  the  nature  of  our  universe  may  be 
intermediate  between  Einstein's  "  spherical  universe/'  con- 
taining matter,  and  the  "  expanding  world  "  of  de  Sitter,  de- 
void of  matter.  According  to  the  theories  of  Lemaitre,  as 
developed  by  Eddington,1  the  universe  must  be  expanding  at 
a  rate  which  is  constantly  increasing  and  ultimately  the 
nebulae  must  be  separating  at  a  speed  greater  than  that  of 
light  itself.  Under  these  circumstances,  they  will  be  invisible 
to  us  and  will  be  "  below  our  horizon."  Possibly  a  great  part 
of  the  universe  is  already  below  our  horizon. 

The  great  difficulty  is  that  the  enormous  velocities  of  the 
nebulae  now  observed  permit  a  time  scale  of  a  mere  io10 
years.  At  the  expiration  of  this  time,  matter  will  cease  to 
exist.  We  might  overcome  the  difficulty  involved  in  the 
theory  of  the  expanding  universe  by  assuming  (since  all  mo- 
tions are  relative)  that  we  ourselves  are  becoming  smaller 
and  smaller.  On  this  assumption,  our  terrestrial  yardsticks, 
the  earth  and  the  moon  and  the  sun,  and  even  the  stars  and 
nebulae  of  our  galactic  system,  are  dwindling  in  size  in  the 
same  progression  and  will  ultimately  cease  to  exist. 

Unfortunately,  the  time  scale  of  years,  colossal  though  it 
be  when  compared  with  the  span  of  human  life,  is  far  too  small 
to  explain  the  life  of  the  stars,  or  even  of  that  star  of  great 
importance  to  us,  the  sun. 

In  an  important  paper  2  written  jointly  by  Einstein  and  de 
Sitter,  the  conclusion  is  reached  that  an  increase  in  the  pre- 
cision of  the  observations  may  enable  us  in  the  future  to  de- 
termine the  density  of  matter  in  the  universe,  and  to  fix  the 
sign  and  determine  the  value  of  the  radius  of  curvature.  Un- 
fortunately, any  further  discussion  of  these  problems  now 
of  such  vital  interest  to  astronomers  will  carry  us  too  far  afield. 

1  Monthly  Notices,  R.  A.  S.,  gi,  413,  1931. 

2  Proc.  Nat.  Acad.  Sciences,  i8}  213,  1932. 


CHAPTER  XXIV 

SHADOW  BANDS 

SHADOW  bands  have  no  astrophysical  importance  and 
for  that  reason  they  will  be  treated  here  very  briefly. 
They  are  seen  at  the  beginning  and  at  the  end  of  the 
total  phase  of  a  solar  eclipse,  and  they  should,  perhaps,  be 
mentioned  in  a  treatise  on  eclipses  even  though  they  involve 
meteorological  rather  than  astrophysical  problems. 

The  most  complete  observations  of  shadow  bands  at  an 
eclipse  are  those  made  on  January  24,  1925.  The  reason  is 
that  snow  covered  the  ground  throughout  the  whole  of  the 
belt  of  totality  with  the  result  that  each  and  every  observer 
was  furnished  with  a  white  sheet  on  which  the  bands  were 
portrayed.  Excellent  descriptions  of  the  observations  are 
given  by  Bassett.1 

The  1925  bands  were  probably  intensified  due  to  the  fact 
that  the  time  of  the  eclipse  was  early  in  the  morning  when 
convection  was  active  but  probably  confined  to  the  lower 
layers  of  the  atmosphere,  within  300  yards  of  the  earth. 

The  general  features  of  the  observations  agree  with  those 
of  earlier  eclipses.  The  shadow  bands  were  seen  for  two 
or  more  minutes  before  totality  and  the  same  length  of  time 
after  totality.  Some  observers  reported  seeing  bands  twenty 
minutes  before  totality,  but  such  bands  were  probably  caused 
by  heated  air  rising  from  furnace  chimneys,  and  in  several 
cases  this  was  proven  at  the  time  to  be  the  cause.  No  ob- 
servers regarded  to  be  reliable  reported  the  shadow  bands 
during  totality. 

The  bright  bands  were  generally  observed  to  be  broken  and 
confused  arcs  or  short  blotches  of  light,  a  third,  a  half,  or  even 
a  full  meter  in  length.  The  dark  band  was  usually  reported 
as  having  a  width  one-fifth  that  of  the  bright  band,  the  dark 

1  Popular  Astronomy,  33,  232,  1925. 
480 


SHADOW  BANDS  481 

band  being  observed  by  different  persons  from  £  inch  to 
2  inches  in  width,  while  the  bright  areas  had  a  width  varying 
from  6  inches  to  i  yard. 

Naturally,  in  the  estimation  of  the  speed  of  travel  of  the 
shadow  bands,  there  was  a  wide  range  from  stationary  with 
a  shimmering  light,  to  very  fast  motions  of  30  miles  per  hour. 

The  general  consensus  of  opinion  of  the  very  large  num- 
ber of  observers  taking  part  was  that  the  shadow  bands  were 
narrowest,  most  distinct,  and  most  sharply  defined  immedi- 
ately before  and  after  totality,  and  that  they  became  broader, 
farther  apart,  and  more  indistinct  until  they  disappeared, 
with  increase  of  width  of  the  solar  crescent. 

At  the  October,  1930,  eclipse,  shadow  bands1  were  ob- 
served. They  were  first  seen  about  one  minute  before  to- 
tality and  were  visible  for  about  twenty  seconds.  The  dark 
bands  were  about  i^  inches  in  width  and  were  separated 
6  inches.  Their  motion  was  at  the  rate  of  10  miles  per  hour 
in  a  west  by  north  direction  which  had  no  connection  with  the 
direction  of  the  wind  at  the  time.  They  were  seen  also  after 
totality  but  not  during  totality. 

Hastings  pointed  out  many  years  ago  that  the  motions  of 
the  shadow  bands  must  be  perpendicular  to  their  lengths, 
otherwise  the  motions  could  not  be  observed,  and  that  they 
must  lie  approximately  parallel  to  the  crescent  of  the  sun. 
The  shadow  bands  therefore  can  have  no  connection  with  the 
direction  of  the  wind,  a  fact  which  all  observations  seem  to 
confirm. 

Most  authorities  agree  in  believing  that  the  shadow  bands 
are  caused  by  irregularities  in  the  optical  density  of  the 
atmosphere,  the  crescent  character  of  the  sun  magnifying  the 
effects  of  contrast  in  the  bands. 

In  Handbuch  der  Astrophysik,  4, 353,  1929,  a  detailed  dis- 
cussion is  given.  As  shown  there,  Humphreys  considered 
four  theories  for  explaining  the  bands,  only  one  of  which  will 
be  mentioned  here. 

The  heated  air  commonly  rises  from  the  earth  in  numerous 
isolated  masses,  and  experience  teaches  us  that  when  this 
lighter  air  ascends  in  a  connected  column  a  whirlwind  is  es- 

1  Popular  Astronomy,  39,  241.  1031. 


482  ECLIPSES  OF  THE  SUN 

tablished.  The  passage  of  light  from  the  denser  to  the  rarer 
air  may  cause  total  reflection  over  a  small  area  of  the  inter- 
face. In  reality  there  is  a  deflectional  bending  of  the  rays 
as  they  pass  through  the  shell  exactly  in  the  manner  of  the 
inferior  mirage,  so  commonly  seen  in  summer  over  heated 
streets  and  smooth  roads.  All  the  light  from  every  object 
included  within  the  angle  of  total  reflection  would  be  wholly 
excluded  from  that  place  on  the  surface  of  the  earth  which 
it  otherwise  would  have  reached,  and  the  light  thus  excluded 
would  be  added  to  that  in  the  adjoining  place. 

In  this  way,  the  crescent  of  the  sun  a  few  minutes  before 
and  after  totality  must  produce  narrow  dark  bands  which  be- 
come more  distinct  and  more  numerous,  since  smaller  in- 
homogeneities  would  then  suffice,  as  totality  is  approached. 
Each  dark  band  should  be  bordered  on  one  side  by  a  narrow 
strip  of  extra  brightness.  The  dark  band  with  bright  border 
would  be  much  narrower  than  that  of  the  intermediary 
lighted  strip  between  successive  bands.  Moreover,  there 
would  be  no  uniformity  in  either  the  widths  of  the  bands  or 
the  distances  between  them.  The  bright  edge  of  the  dark 
band  would  probably  be  difficult  to  observe  on  account  of 
the  shimmering  which  is  constantly  in  play. 

Apparently,  the  shadow  bands  are  caused  by  pseudo-total 
reflections,  or  mirage  effects,  produced  by  the  transition 
shells  between  warmer  and  cooler  adjacent  masses  of  air  in  a 
state  of  thermal  convection. 


CHAPTER  XXV 

THE  ECLIPSES  OF  1932  AND  1934 


F  I  ^HE  great  majority  of  eclipse  astronomers  who  at- 
I  tempted  observations  on  August  31,1932,  succeeded 

JL  only  in  witnessing  sky  conditions  as  if  sunset,  mid- 
night and  dawn  had  all  come  within  the  brief  compass  of  two 
minutes  of  time. 

The  eclipse  took  place  in  the  midst  of  the  holiday  season  of 
the  year  along  an  easily  accessible  track  passing  through  a 
beautiful  section  of  the  country.  Moreover,  the  meetings  of 
the  International  Astronomical  Union  had  been  timed  to  be- 
gin, in  Cambridge,  Massachusetts,  two  days  after  the  eclipse 
had  occurred.  The  result  was  that  more  astronomers  at- 
tempted to  see  and  to  observe  this  phenomenon  than  ever  be- 
fore in  eclipse  history.  Every  few  miles  along  the  moon's 
shadow  path,  from  Parent  away  north  of  the  St.  Lawrence 
River  in  Quebec  and  eastward  to  the  tip  of  Cape  Cod,  there 
were  expeditions  of  all  sizes  and  descriptions,  from  amateur 
equipment  of  small  dimensions  to  the  largest  and  most  refined 
of  professional  instruments,  cameras,  spectrographs,  pho- 
tometers, polarimeters,  etc. 

As  the  eclipse  track  lay  in  the  St.  Lawrence  River  Valley, 
the  expectations  for  clear  skies  were  none  too  high.  The  areas 
of  low  barometric  pressures  which  bring  clouds  and  rain  in 
their  passage  eastward  over  the  great  American  continent 
seem  to  acquire  the  habit  of  passing  out  through  the  St.  Law- 
rence Valley.  The  weather  records  for  five  years  carefully 
assembled  for  the  purpose  seemed  to  show  that  the  mountains 
in  Vermont  and  New  Hampshire  possibly  might  attract  clouds 
and  the  Atlantic  Ocean  bring  fog.  In  planning  the  details  of 
an  eclipse  expedition,  an  observer  of  eclipses  must  be  a  con- 
firmed optimist  in  hoping  always  for  clear  skies.  The  three 
astronomers,  each  with  several  eclipses  to  his  credit,  who  ap- 

483 


484  THE  ECLIPSES  OF  1932  AND  1934 

pear  to  have  had  the  best  average  of  luck  with  clear  skies  at 
eclipses  in  the  past  were  my  friends  Campbell,  Miller  and 
myself.  As  Campbell  was  not  to  observe  the  1932  eclipse, 
the  interesting  question  was,  would  it  be  a  wise  plan  to  locate 
near  Miller  or  myself?  Or  would  the  law  of  averages  work 
out  with  the  result  that  we  both  should  be  avoided?  Of 
course,  the  careful  scientist  never  believes  in  luck! 

Unfortunately,  the  weather  experienced  at  eclipse  time, 
particularly  near  totality,  did  not  live  up  to  the  fifty-fifty  ex- 
pectations, clouds  being  the  rule  rather  than  the  exception. 
In  fact,  the  skies  were  pretty  generally  overcast  throughout 
the  whole  of  the  eclipse  path  except  for  a  hole  here  and  there 
through  the  clouds  near  where  the  sun  was  found  at  totality. 
A  partly  clear  patch  greeted  the  expedition  from  Greenwich 
Observatory  at  Parent.  Similar  conditions,  a  little  better  or 
a  little  worse  were  experienced  as  far  east  as  the  St.  Lawrence 
River.  It  was  cloudy  in  Montreal  and  cloudy  generally  east- 
ward where  there  were  large  well-equipped  expeditions 
throughout  Quebec,  Vermont  and  New  Hampshire.  At  the 
boundary  between  New  Hampshire  and  Maine  a  few  clear 
patches  of  sky  brought  partial  success  to  parties  from  Lick, 
Michigan,  Van  Vleck  and  Georgetown  Observatories.  Nearer 
the  Atlantic,  where  conditions  promised  less,  were  found  the 
clearest  skies  of  all. 

As  a  whole,  the  well-prepared  plans  of  the  various  expe- 
ditions accomplished  comparatively  little,  though  success 
through  propitious  skies  was  met  here  and  there.  Perhaps 
with  the  irony  of  fate,  many  of  the  astronomers  assembling  for 
the  meetings  of  the  International  Astronomical  Union  who 
had  little  or  no  eclipse  experience  and  who  wished  merely  to 
see  the  gorgeous  spectacle  were  given  clear  skies,  though  at 
times  rapid  trips  in  automobiles  had  to  be  made  in  directions 
towards  which  the  clouds  appeared  to  be  breaking,  while  vet- 
eran observers  with  refined  equipment  and  big  programs  were 
doomed  to  disappointment.  The  largest  group  of  instru- 
ments at  any  one  location  was  at  Magog,  100  miles  east  of 
Montreal.  Here  were  the  British  party  under  the  direction  of 
Professor  Stratton  of  Cambridge,  a  Canadian  party  from 
McGill  University,  Dr.  Minnaert  from  Holland  and  an  Ameri- 


I 


5  § 


83 


MOON'S  SHADOW  IN  1932  AT  FIVE-SECOND  INTERVALS 
Photographed  by  Captain  A.  W.  Stevens  from  National  Geographic  Magazine. 


THE  ECLIPSES  OF  1932  AND  1934  485 

can  expedition  from  the  Leander  McCormick  Observatory. 
Four  nations  were  represented,  a  total  of  thirty  instruments 
and  sixty  astronomers  on  eclipse  day.  Many  of  us  had  been 
there  a  month  living  at  the  Hermitage  Country  Club.  On  the 
rough  ground  between  two  fairways  of  the  golf  course,  we  had 
wielded  pick  and  shovel,  and  hammer  and  saw.  We  had  been 
civil  engineer,  electrical  engineer,  photographer,  physicist 
and  astronomer  all  rolled  into  one. 

On  eclipse  day  clear  skies  remained  with  us  all  morning. 
At  noon  clouds  began  to  gather.  Would  they  clear  away  in 
the  hours  before  totality?  Unfortunately,  as  time  pro- 
gressed, conditions  went  from  bad  to  worse.  First  contact 
was  observed  through  heavy  clouds.  At  times  a  little  thinning 
of  the  clouds  appeared  now  and  then.  Two  minutes  before 
totality,  the  sun's  crescent  appeared  which  permitted  the  solar 
beam  to  be  adjusted  into  position  in  the  powerful  concave 
grating  spectrographs.  Would  this  semi-clear  spot  expand 
and  bring  success  or  would  it  close  up  and  bring  failure?  No 
matter  what  the  conditions,  we  decided  to  carry  through  our 
programs.  I  was  to  give  the  signal  "  Go!  "  for  the  combined 
eclipse  camps.  At  the  calculated  time  of  totality,  the  sun's 
crescent  was  still  visible.  Three  seconds  later  this  had  disap- 
peared and  the  signal  "  Go!  "  rang  out.  Totality  had  come, 
and  the  steady  count  was  heard :  "  one,"  "  two,"  "  three."  .  .  . 
There  was  a  clear  patch  of  sky  directly  overhead  but  there  was 
nothing  but  heavy  clouds  in  the  direction  towards  the  eclipsed 
sun.  I  in  Quebec,  and,  a  little  farther  along  the  eclipse  track 
across  the  international  boundary  in  northern  Vermont,  my 
good  friend  Miller  were  both  experiencing  the  thickest  clouds 
either  had  ever  met  in  the  combined  seventeen  eclipses  wit- 
nessed. It  has  so  happened  that  we  have  never  observed  the 
same  eclipse  at  the  same  spot  and  in  1932  we  were  located 
closer  together  than  ever  before. 

It  would  serve  no  useful  purpose  to  attempt  to  give  here  a 
detailed  account  of  the  many  expeditions  in  the  field  or  the 
programs  of  observations  attempted.  Those  interested  may 
readily  find  this  information  from  the  contemporary  astro- 
nomical journals  like  Popular  Astronomy,  Journal  of  the 
Royal  Astronomical  Society  of  Canada,  Publication  of  the 


486  THE  ECLIPSES  OF  1932  AND  1934 

Astronomical  Society  of  the  Pacific,  Monthly  Notices,  etc. 
An  attempt  will  be  made  merely  to  give  the  outstanding  ac- 
complishments, arranging  these  from  northwest  to  southeast 
as  the  eclipse  passed  over  the  country. 

Farthest  north  was  the  party  from  Greenwich  Observatory 
at  Parent.  On  eclipse  day  it  was  cloudy  practically  continu- 
ously. Twice  during  the  morning  the  sun  was  glimpsed  and 
an  opportunity  was  given  for  checking  the  position  of  the 
45-foot  camera.  After  noon  there  were  one  or  two  breaks  in 
the  clouds,  one  of  them  about  five  minutes  before  totality. 
"  The  corona  x  was  seen  before  second  contact,  but  the  clouds 
were  thickening,  and  after  mid-totality  nothing  was  visible 
until  near  the  end  when  prominences  and  corona  were  again 
faintly  visible.  .  .  .  When  the  eclipse  was  over  and  before 
the  photographs  were  developed,  it  was  expected  that  no  re- 
sults of  value  had  been  obtained."  Fortunately,  the  photo- 
graphs with  the  45-foot  camera  and  2  i-foot  prismatic  camera 
far  exceeded  expectations. 

A  reproduction  of  the  flash  spectrum  at  second  contact 
given  facing  page  492  shows  a  sudden  termination  of  the  cres- 
cent lines  of  the  spectrum  particularly  at  the  upper  ends  of  the 
arcs.  The  direct  photographs  show  a  distinct  flattening  of  the 
edge  of  the  moon.  To  prove  that  this  effect  was  real  and  not 
apparent  caused  by  irradiation,  other  photographs  were  meas- 
ured. It  was  concluded  that "  there  was  a  depression  amount- 
ing to  2"  extending  about  8°  on  each  side  of  position  angle 
132  °  —  the  point  of  second  contact."  On  account  of  the  in- 
creasing density  of  the  clouds,  the  photographs  taken  toward 
the  end  of  totality  showed  nothing. 

Jackson  observed  second  contact  visually  three  or  four  sec- 
onds later  than  the  corrected  time  furnished  by  the  director 
of  the  American  Ephemeris  shortly  before  the  eclipse,  while 
the  observed  duration  of  totality  was  99  sees,  against  102  sees, 
calculated.  The  retardation  of  second  contact  and  the  shorter 
duration  of  totality  are  both  readily  explained  by  the  flat- 
tening of  the  moon's  eastern  limb. 

Concerning  the  photograph  of  the  flash  spectrum,  it  will  be 
best  to  let  the  Greenwich  astronomers,  Davidson  and  Jack- 

1  Monthly  Notices,  03,  3,  1932. 


THE  ECLIPSES  OF  1932  AND  1934  487 

son,  speak  for  themselves  (loc.  cit.}.  "  The  photograph  taken 
at  the  first  flash  is  very  good.  The  lines  are  numerous  and  in 
good  focus  from  3590  A  to  the  limit  of  sensitivity  of  the  pan- 
chromatic plate,  the  helium  line  at  7065  A  being  shown.  It  is 
not  considered,  however,  that  it  will  show  any  lines  which 
have  not  been  photographed  by  one  or  another  of  the  past 
observers  of  flash  spectra.  Although  heights  might  be  de- 
duced from  the  lengths  of  the  arcs,  these  would  be  very  un- 
certain for  the  lower  lines,  on  account  of  the  irregularity  of  the 
moon  near  the  point  of  second  contact.  Many  high  level  lines 
which  are  visible  30°  from  the  geometrical  point  of  contact 
are  invisible  20°  from  it. 

"  The  objective  prism  photographs  were  taken  to  show  the 
distribution  of  the  various  elements  in  the  chromosphere  and 
prominences,  and  for  comparison  with  the  direct  photographs 
of  the  corona  secured  with  the  large  coronagraph.  As  regards 
wave-length  determinations  and  the  identifications  of  the  ele- 
ments in  the  chromosphere,  the  best  spectra  already  secured 
with  objective  prisms  and  gratings  or  with  slit  spectrographs 
cannot  greatly  be  improved  on  in  the  region  covered  by  the 
panchromatic  plate  except  with  more  powerful  instruments 
under  favorable  circumstances." 

The  spectrum  photograph  immediately  following  the  first 
flash  showed  the  high  level  chromospheric  lines  and  also  the 
coronal  lines  at  5303  A  in  the  green  and  6374  A  in  the  red. 
The  latter  line  is  shown  as  a  nearly  complete  ring  of  practi- 
cally uniform  intensity  while  the  green  line  has  its  light  con- 
centrated near  prominences.  The  1932  eclipse  thus  confirms 
the  conclusions  found  on  page  380  that  these  two  strong  co- 
ronal lines  do  not  have  a  common  origin. 

In  addition  to  the  excellent  spectra,  good  photographs  were 
obtained  with  the  45-foot  camera  used  with  coelostat  showing 
the  prominences  and  the  brighter  parts  of  the  inner  corona. 

The  holes  in  the  clouds  were  more  favorably  placed  near  the 
St.  Lawrence  River  especially  at  Louiseville  and  Actonvale. 
The  most  powerful  equipment  in  this  region  was  that  of  the 
French  party  under  the  direction  of  the  veteran  eclipse  ob- 
server, de  la  Baume  Pluvinel.  The  clouds  broke  immediately 
before  totality,  permitting  valuable  observations  on  polariza- 


488  THE  ECLIPSES  OF  1932  AND  1934 

tion  by  Dufay  and  Grouiller,  to  which  reference  is  made  on 
page  509.  Not  far  away,  parties  consisting  largely  of  ama- 
teurs organized  by  the  Royal  Astronomical  Society  of  Canada 
made  direct  photographs  of  the  corona  and  also  observations 
to  locate  the  edge  of  the  moon's  shadow  path. 

At  Montreal,  and  south  and  east  of  the  St.  Lawrence 
through  Quebec,  Vermont  and  most  of  New  Hampshire  where 
expeditions  were  located,  the  clouds  permitted  at  best  only 
fleeting  glimpses  of  the  corona.  East  of  the  White  Mountains 
at  North  Con  way,  the  clouds  were  just  about  as  thick  as  those 
experienced  south  from  Montreal.  A  few  miles  farther  south- 
east at  Center  Conway  and  Fryeburg,  totality  began  through 
clouds  but  fortunately  conditions  greatly  improved,  though 
thin  clouds  still  persisted  to  the  end  of  totality.  Under  such 
conditions,  happening  during  the  excitement  of  a  most  un- 
usual event,  it  is  impossible  for  the  eclipse  observer  to  know 
whether  the  clouds  have  actually  thinned  or  whether  the  ap- 
parent improvement  results  only  from  the  fact  that  there  is 
no  strong  sunlight  shining  on  the  clouds  to  make  them  visible. 
Few  astronomers  have  had  experience  sufficient  to  serve  as  a 
guide  —  but  the  developed  photographs  tell  the  tale.  For- 
tunately, the  corona  was  photographed  in  excellent  detail  by 
the  Lick  expedition  (reproduced  facing  page  476)  and  by  the 
Van  Vleck,  Georgetown  and  University  of  Michigan  expedi- 
tions. Closer  to  the  Atlantic  Ocean,  in  the  state  of  Maine,  the 
weather  defied  all  rules  with  the  result  that  conditions  were 
much  better  than  expected.  At  Limerick,  the  U.  S.  Naval 
Observatory  party  secured  successful  photographs  of  the  co- 
rona through  thin  cirrus  clouds  with  65-foot,  1 5-foot,  and  with 
38-  and  33-inch  cameras.  At  Alfred,  conditions  were  even 
better,  permitting  the  expedition  from  Japan  to  obtain  pho- 
tographs of  the  corona  and  flash  spectrum.  The  Harvard 
party  at  Gray  had  the  best  luck  of  all  the  expeditions  in  the 
field.  Thin  cirrus  clouds  were  present  until  noon,  but  a  per- 
fectly clear  sky  prevailed  throughout  the  whole  of  the  eclipse. 
The  program  was  chiefly  photometric  and  was  concerned  with 
measuring  the  relative  proportions  of  scattering  and  polariza- 
tion in  the  corona.  The  work  was  done  both  visually  and  pho- 
tographically, plates  being  secured  in  six  different  spectral 


THE  ECLIPSES  OF  1932  AND  1934  489 

regions  by  using  appropriate  filters.  Three  out  of  four  pin- 
hole  photometers  at  different  localities  were  operated  under 
clear  skies. 

The  Lick-Crocker  expedition  *  at  Fryeburg,  with  interest- 
ing spectroscopic  programs  planned  with  powerful  and  va- 
ried equipment,  fortunately  had  good  luck  with  the  weather. 
A  total  of  ten  spectrographs  were  employed,  four  for  the 
chromosphere  and  six  for  the  corona.  Although  the  sky  was 
never  clear,  conditions  at  the  end  of  totality  were  much  better 
than  at  the  beginning,  with  the  result  that  the  Lick  spectra, 
particularly  those  of  the  chromosphere,  supplement  those  of 
the  Greenwich  party  far  north  at  Parent  where  the  clearest 
sky  was  experienced  at  the  beginning  of  totality. 

For  photographing  the  flash  spectrum  without  slit,  two 
methods  are  described  in  Chapter  XV,  with  fixed  and  with 
moving  plate.  Each  method  has  its  advantages  and  its  dis- 
advantages. At  Fryeburg  it  was  wisely  decided  to  try  both 
methods.  In  order  to  photograph  as  great  a  range  of  wave- 
lengths as  possible,  two  spectrographs  were  used  with  both 
moving  and  fixed  plates.  Experience  at  former  eclipses  has 
shown  that  with  an  objective  prism,  well-timed  chromospheric 
spectra  may  be  secured  with  exposures  of  one  second  dura- 
tion. In  place  of  the  usual  type  of  objective  spectrograph 
with  fixed  plate  heretofore  used  at  eclipses,  a  new  instrument 
(loc.  aY.)  was  designed  for  use  at  Fryeburg  called  a  "  jump- 
ing film  spectrograph. "  Starting  one  minute  before  totality, 
exposures  each  of  one  second  were  made  every  alternate  sec- 
ond. With  a  break  for  one  long  exposure  lasting  30  seconds 
at  mid-totality,  the  film  was  continued  in  its  steady  and  jump 
motion  until  one  minute  after  totality. 

It  is  evident  that  with  a  long  series  of  successful  photo- 
graphs taken  with  a  jumping  film,  heights  in  the  chromo- 
sphere can  be  obtained  by  two  methods:  by  measuring  the 
angular  lengths  of  the  chromospheric  arcs  or  by  noting  the 
appearance  or  disappearance  of  spectral  lines  on  successive 
photographs.  This  mechanical  method  of  exposure  has  dis- 
tinct advantages  over  that  always  followed  by  the  writer  of 
observing  with  a  direct-vision  spectroscope  and  timing  the  ex- 

1  Publ.A.S.P.,44,34i,  IQ32. 


490  THE  ECLIPSES  OF  1932  AND  1934 

posures  for  the  flash  spectrum  accordingly.  At  Fryeburg,  the 
mechanism  and  the  observer  did  their  work  as  planned,  with 
beautiful  photographs  as  the  result,  though  thin  clouds  of 
variable  thickness  unfortunately  covered  the  eclipsed  sun. 

Measures  of  the  flash  spectrum  at  previous  eclipses  have 
shown  that  one-quarter  of  all  chromospheric  crescents  visible 
on  the  best  photographs  extend  to  the  moderate  heights  of 
300  kms  or  less  above  the  photosphere  and  hence  are  covered 
in  one  second  of  time  by  the  advancing  moon;  two-thirds  of 
all  lines  of  the  flash  spectrum  are  covered  in  the  brief  interval 
of  one  second  and  a  half.  Evidently,  if  the  one  second  visi- 
bility for  the  low-lying  lines  happened  to  exactly  coincide  with 
the  exposure  time  of  one  second  given  by  the  jumping  film 
spectrograph,  the  observer  should  be  congratulated  on  his 
rare  good  luck.  If,  on  the  other  hand,  the  film  was  being 
changed  during  this  one  particular  second,  the  developed  pho- 
tographs would  show  none  of  the  lines  of  the  lowest  level.  As 
either  of  these  two  methods  of  exposure  are  equally  probable, 
it  is  obvious  that  this  hit-and-miss  method  of  making  expo- 
sures, on  the  average  can  not  photograph  the  low-level  lines; 
also,  notwithstanding  the  quick  succession  of  photographs, 
the  observer  will  not  know,  and  the  photographs  can  not  show, 
just  when  the  effective  existence  of  the  several  chromospheric 
strata  began  and  ended. 

The  spectra  taken  by  the  jumping  film  at  Fryeburg  are  su- 
perior in  quality  to  those  taken  with  the  moving  plate.  Fac- 
ing page  493  *s  a  reproduction  of  the  flash  spectrum  taken  at 
the  end  of  totality  by  Menzel.  The  very  rough  character  of 
the  moon's  edge  shown  by  this  photograph  will  make  the  de- 
termination of  heights,  particularly  for  lines  of  low  level,  a 
difficult  problem.  It  is  unfortunate  that  at  the  1932  eclipse, 
the  limbs  of  the  moon  both  at  second  and  third  contacts  were 
so  jagged.  The  long  exposure  spectrum  taken  at  mid- totality 
shows  the  green  and  red  coronal  lines  with  distributions  of 
radiation  exactly  similar  in  character  to  that  found  on  the 
Greenwich  plates. 

For  the  corona,  six  spectrographs  were  used  by  the  Lick 
party.  Three  instruments  with  slit  placed  east  and  west  of 
one,  two  and  three  prisms,  respectively,  permitted  spectro- 


THE  ECLIPSES  OF  1932  AND  1934  491 

grams  of  excellent  definition  which  record  the  spectrum  of  the 
corona,  especially  that  of  its  brighter  parts,  in  great  strength. 
A  marked  feature  of  the  three  is  the  great  intensity  of  the  con- 
tinuous spectrum  of  the  inner  corona  and  the  number  and 
strength  of  the  prominence  lines.  Only  the  stronger  emission 
lines  are  visible.  For  photographing  in  the  red,  two  grating 
spectrographs  were  used,  one  of  concave,  the  other  of  plane 
grating.  With  these  no  results  were  obtained.  The  sixth  was 
an  interferometer  spectrograph  for  the  wave-length  of  the 
green  line  with  which  successful  photographs  were  obtained. 
The  results  obtained  from  measures  of  these  spectra  are  dis- 
cussed on  page  502. 

Also  at  Fryeburg,  the  University  of  Michigan  expedition 
secured  excellent  photographs  of  the  corona  with  a  4o-foot 
focus  camera  pointed  directly  at  the  sun  and  also  of  the  flash 
spectrum  with  two-prism  dispersion.  In  addition,  large-scale 
movie  photographs  were  secured  by  the  McNath-Hulbert 
branch  of  the  University  Observatory  with  cameras  of  74,  40 
and  14  inches  focus,  respectively.  Nearby,  the  party  from  the 
Van  Vleck  Observatory  successfully  photographed  the  corona 
with  25-foot  focus  and  with  smaller  cameras.  Photographs 
obtained  by  these  two  expeditions  are  given  in  Popular  As- 
tronomy for  October,  1 93  2 .  A  beautiful  photograph  obtained 
by  the  Georgetown  Observatory  expedition  is  reproduced  in 
the  National  Geographic  Magazine,  57,  600,  1932. 

As  was  predicted  on  page  404,  the  corona  was  of  the  mini- 
mum type  with  long  equatorial  streamers  and  pronounced 
polar  brushes. 

The  distinguished  air-pilot,  Captain  Albert  W.  Stevens, 
writes  a  thrilling  account x  of  photographing  the  moon's 
shadow  at  a  height  of  five  miles  above  the  earth.  Facing 
page  485  are  two  of  his  remarkable  photographs  showing  the 
motion  of  the  edge  of  the  moon's  shadow  as  projected  on  the 
clouds  at  5-second  intervals.  The  end  of  totality  is  described 
in  the  following  words:  "  At  this  time  the  most  remarkable 
sight,  except  the  corona  itself,  was  the  horizon,  beyond  the 
shadow,  to  the  northwest.  The  far  edge  of  the  shadow  was 
defined  by  a  narrow  band  of  a  saffron  color  and  was  distinctly 

1  National  Geographic  Magazine,  57,  581,  1932. 


492  THE  ECLIPSES  OF  1932  AND  1934 

seen  to  be  curved.  The  air  between  seemed  to  be  of  an  indigo 
color.  Under  the  airplane,  through  openings  in  the  clouds, 
the  earth's  surface,  five  miles  below,  could  be  dimly  seen." 

In  addition  to  the  observations  made  by  the  U.  S.  Army 
pilot,  the  U.  S.  Navy  also  photographed  *  the  moon's  shadow 
above  the  clouds.  Among  the  other  observers  from  airplanes, 
mention  will  be  made  only  of  Clyde  Fisher  who  states,2  "  The 
shadow  of  the  moon  was  first  seen,  just  a  little  before  totality, 
as  a  narrow  band  on  this  level  sea  of  clouds,  at  a  distance  of 
probably  more  than  one  hundred  miles.  As  it  approached  it 
became  less  distinct,  less  definite.  .  .  .  Upon  development, 
the  picture  seems  to  be  well  timed  and  it  shows  just  enough 
diminution  of  light  to  indicate  the  passing  of  the  moon's 
shadow.  However,  no  edge  is  to  be  seen  in  the  pictures,  and 
none  was  seen  in  our  observation." 

Thermometers  were  read  at  more  than  200  stations  in  order 
to  record  the  drop  in  temperature  during  the  progress  of  the 
eclipse,  the  work  being  organized  by  the  Blue  Hill  Meteoro- 
logical Observatory.3  The  widespread  clouds  which  existed 
at  five  different  levels  and  played  such  havoc  with  the  astro- 
nomical observations  were  a  blessing  in  disguise  for  the  tem- 
perature records.  It  was  found  that  temperatures  fell  from 
2  to  ii  degrees  F.,  the  smaller  values  occurring  where  clouds 
were  dense,  the  larger  where  the  weather  was  clear  and  the 
surface  of  the  ground  sandy.  With  the  assistance  of  three 
U.  S.  Navy  airplanes,  temperature  records  were  made  at 
Fryeburg  4  at  eight  different  levels  above  the  ground  at  alti- 
tudes of  5,  25,  50,  75,  100,  866,  3261  and  10122  feet,  respec- 
tively. The  observations  show  ( i )  that  the  greatest  range  in 
temperature  occurred  at  the  earth's  surface,  (2)  that  the 
ranges  decreased  rapidly  with  altitude  and  (3)  that  the  effect 
of  atmospheric  refraction  upon  the  deflection  of  starlight  by 
the  sun  during  an  eclipse  is  entirely  negligible. 

The  total  eclipse  of  February  14,  1934  was  successfully  ob- 
served 5  from  two  small  coral  reefs,  Losap  and  Laol,  in  the 

1  Popular  Astronomy,  41,  202,  1933. 

2  Ibid.,  40,  467,  1932. 

3  Bull.  Amer.  Meteor ol.  Socy.,  13,  159,  1932. 

4  Annals  of  the  Dearborn  Observatory,  4,  27,  1933. 

5  Popular  Astronomy,  42,  307,  1934. 


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THE  ECLIPSES  OF  1932  AND  1934  493 

far-off  Caroline  Islands.  The  National  Research  Council  had 
been  authorized  by  the  Imperial  Japanese  government  to  in- 
vite astronomers  the  world  over  to  be  the  guests  of  Japan  for 
the  observation  of  the  eclipse.  Only  two  foreigners,  however, 
availed  themselves  of  this  generous  offer,  Dr.  Willi  M.  Cohn 
and  J.  J.  Johnson,  both  of  California.  The  Cruiser  "  Ka- 
suga  "  of  the  Imperial  Japanese  Navy  conveyed  the  eclipse 
party  on  its  eight-day  voyage  from  Yokohama.  Three  busy 
weeks  were  spent  on  the  islands  in  preparation  for  the  great 
event.  The  largest  part  of  the  expedition  was  from  the 
Tokyo  Imperial  University  under  the  direction  of  Dr.  Satome. 
A  well-planned  series  of  observations  was  carried  out  covering 
the  most  important  problems  dealing  with  eclipses.  Special 
stress  was  placed  on  securing  photographs  to  test  the  rela- 
tivity displacement  of  star  images,  the  1934  eclipse  being 
specially  favorable  l  because  the  sun  was  projected  against  an 
unusually  rich  field  of  stars.  In  fact,  for  the  next  two  decades 
there  will  not  be  as  favorable  a  star  field  for  measuring  the  so- 
called  Einstein  displacement.  The  party  from  the  Kyoto 
Imperial  University  duplicated  most  of  the  work  on  the 
Tokyo  program,  Dr.  Ueta  secured  the  Einstein  plates  and 
large-scale  photographs  of  the  corona. 

Dr.  Cohn  used  several  cameras  on  a  polar  axis  for  measur- 
ing the  polarization  of  the  sky  near  the  eclipsed  sun.  Visual 
observations  were  also  made.  Johnson's  observations  were 
visual  with  a  modified  Lyot-type  polarimeter. 

As  is  usually  the  case  with  eclipse  expeditions  in  the  tropics, 
there  was  a  superabundance  of  clouds  and  rain  during  the 
weeks  of  preparation.  The  morning  of  eclipse  day  brought  a 
great  surprise.  In  describing  the  eclipse,  Johnson  writes, 
"  It  was  our  good  fortune  to  observe  one  of  nature's  rarest 
treats,  a  total  eclipse  of  the  sun  under  absolutely  perfect  con- 
ditions: clear  blue  sky  for  miles  around,  not  the  slightest  sug- 
gestion of  haze  or  mist,  a  steady  atmosphere  —  in  short  the 
kind  of  eclipse  which  the  astronomer  dreams  about  but  sel- 
dom sees.  The  scientific  results  obtained  from  this  latest  to- 
tal edipse  are  discussed  in  the  following  pages. 

American  astronomers  can  look  back  with  great  pride  to  the 

1  Ibid.,  40,  623,  1932. 


494  THE  ECLIPSES  OF  1932  AND  1934 

enviable  record  of  achievements  made  in  eclipse  history  as 
recorded  in  the  foregoing  pages.  Science,  however,  knows  no 
international  boundaries,  with  the  result  that  all  eclipse  as- 
tronomers no  matter  what  their  nationality  look  confidently 
to  the  future.  Who  are  to  be  the  fortunate  individuals  who 
will  observe  the  1936  total  eclipse?  Will  it  be  possible  to  ob- 
serve, under  conditions  that  promise  success,  the  eclipse  of 
1937  with  its  seven-minute  totality?  Will  South  Africa  live 
up  to  its  good  reputation  for  clear  skies  in  1940?  Discoveries 
of  great  importance  to  solar  physics  will  come  only  as  the  re- 
ward of  much  careful  planning  and  concentrated  effort.  In 
the  following  pages  a  brief  summary  is  given  of  the  results 
obtained  from  eclipse  photographs  in  the  past  decade,  or  since 
the  appearance  of  the  first  edition  of  this  book,  and  at  the 
same  time  there  are  outlined  the  main  problems  of  solar  phys- 
ics now  awaiting  solution. 

Since  1923  with  its  disappointing  eclipse,  observations  have 
been  made  in  1925,  1926,  1927,  1929,  1930  (April  28  and 
October  21),  1932  and  1934.  In  the  dozen  years,  there  have 
been  nine  total  eclipses,  one  more  than  the  average  (see  page 
60).  Four  of  the  nine  have  been  visible  in  the  United  States. 
The  weather  conditions  experienced  for  the  nine  eclipses  have 
probably  been  about  average.  The  four  eclipses  observed  in 
the  tropics,  1926,  1929,  1930  (Tin-Can  Island)  and  1934, 
contrary  to  expectations,  experienced  better  average  condi- 
tions of  sky  than  the  balance  of  five  observed  in  temperate 
zones. 

The  spectacular  beauty  of  a  total  eclipse,  observable  at 
times  only  by  traveling,  at  great  expense,  great  distances  from 
home,  brings  in  its  train  a  large  body  of  observers,  profes- 
sional and  amateur  when,  as  in  1932,  the  eclipse  track  is  near 
at  hand.  One  of  the  simplest  problems  to  be  attacked  is  the 
direct  photography  of  the  corona.  The  camera  employed 
may  be  any  size  from  the  short  focal  length  of  a  kodak  to  the 
huge  proportions  used  by  the  expert  eclipse  astronomer,  the 
greatest  length  heretofore  used  being  135  feet  at  the  1900 
eclipse.  For  such  large  dimensions  a  coelostat  is  imperative. 
At  the  1919  eclipse,  the  relativity  photographs  showed  quite 
conclusively  that  coelostat  mirrors  may  bring  distortions. 


THE  ECLIPSES  OF  1932  AND  1934  495 

Hence  the  tendency  in  recent  years  has  been  to  dispense  with 
the  mirror.  For  focal  lengths  greater  than  fifteen  feet,  the 
mounting  has  been  the  tower  telescope.  On  Sproul  Observa- 
tory expeditions  Miller  and  Marriott  have  used  in  this  manner 
focal  lengths  of  63  and  65  feet.  In  my  opinion  the  definition 
secured  at  the  Tin-Can  Island  eclipse  of  1930  had  never  been 
surpassed.  As  is  well  known  to  expert  photographers,  the  de- 
tail appearing  on  plates  of  the  corona  depends  on  securing 
well-timed  exposures  but  also  correct  photographic  develop- 
ment of  the  photographic  plates. 

In  preparing  plans  for  the  1932  eclipse,  Wright x  decided 
to  abandon  the  long-focus  tower  telescope  used  at  Lick  Ob- 
servatory expeditions  since  1893  and  to  employ  instead  the 
two  lenses,  each  of  s-inch  aperture  and  1 5-foot  focal  length, 
used  at  the  1922  eclipse  for  the  relativity  shift.  On  account 
of  the  thin  clouds  experienced  throughout  totality,  the  de- 
cision evidently  was  a  wise  one.  Certainly  the  definition  ob- 
tained by  Wright  in  1932  is  quite  up  to  the  standard  set  by 
Marriott  at  the  1930  eclipse. 

Manifestly,  it  is  much  easier  to  erect  at  the  eclipse  camp, 
sometimes  an  isolated  spot  on  the  globe,  a  polar  axis  brought 
as  a  unit  from  home  with  which  to  mount  a  camera  (or  cam- 
eras) of  is-foot  focus  than  to  build  a  tower  telescope  of  four 
times  this  length,  usually  from  materials  found  near  the 
eclipse  camp  and  frequently  with  native  workmen.  Only 
occasionally  is  it  possible,  as  it  was  in  1930,  to  transport  from 
home  to  a  distant  foreign  country  the  many  thousands  of  feet 
of  lumber  needed  for  the  tower  telescope.  To  build  from  na- 
tive materials  requires  much  experience  and  resourcefulness. 

We  may  indeed  well  ask  ourselves  why  it  is  that  huge  focal 
lengths  have  been  used  to  photograph  the  corona.  Is  it  neces- 
sary to  have  the  diameter  of  the  moon  seven  inches  in  the 
original  photograph?  Why  not  secure  negatives  with  one- 
quarter  the  focal  length  and  then  enlarge  four  times  if  large 
scale  is  desired?  Little  of  detail  in  the  original  photographs 
is  lost  in  the  enlarging  processes  while  on  the  contrary  faint 
coronal  details  may  be  strengthened  and  accentuated.  The 
striving  after  great  focal  lengths  reflects  the  trend  in  all  as- 

1  Publ  A.  S.  P.,  44,  34i> 


496  THE  ECLIPSES  OF  1932  AND  1934 

tronomical  observations  to  use  telescopes  of  greater  and  still 
greater  sizes.  For  observational  work  on  stars,  large  aper- 
tures and  great  focal  lengths  are  necessary  in  order  to  secure 
the  most  refined  results.  With  the  eclipsed  sun  there  is  no 
need  for  great  apertures.  Is  there  need  for  great  scale  in  the 
original  coronal  photographs? 

The  answer  to  this  question  depends  on  many  considera- 
tions, but  primarily  on  the  scientific  uses  to  which  coronal 
photographs  are  to  be  put.  The  relative  costs  of  the  two  pro- 
cedures should  be  taken  into  consideration.  At  the  1932 
eclipse  it  was  a  comparatively  simple  matter  to  put  all  of  the 
eclipse  equipment  on  a  motor  truck  at  Mount  Hamilton,  then 
transport  to  a  steamer  in  San  Francisco  and  then  from  New 
York  to  Fryeburg.  It  was  even  simpler  for  the  Mount  Wilson 
expedition  to  put  all  of  its  equipment  on  a  motor  truck  in 
Pasadena  and  transport  it  directly  across  the  whole  continent, 
a  distance  of  6000  kms,  to  the  eclipse  site  at  Lancaster.  But 
in  the  coming  generation,  American  astronomers  must  travel 
an  average  of  one-third  the  way  around  the  globe  for  each 
total  eclipse.  In  future  eclipses  the  relative  costs  of  trans- 
porting a  heavy  polar  axis  from  home  and  then  erecting  in  the 
field  or  transporting  from  home  the  lighter  equipment  of  the 
tower  telescope  and  then  building  the  double  tower  from  local 
materials  will  differ  little.  The  cost  should  not  be  a  deciding 
factor  —  but  in  most  observatories  it  must  be  considered. 
After  all,  the  problem  is  one  primarily  for  American  observa- 
tories (and  very  few  are  involved)  for  the  reason  that  the 
heavy  polar  axes  to  carry  wide-angle  lenses  to  measure  the 
gravitational  displacement  of  stars  during  totality  and  tower 
telescopes  are  the  contributions  of  America  to  eclipse  history. 

Each  observatory  interested  will  have  to  provide  its  own 
answer  regarding  the  best  equipment  for  it  to  use  for  direct 
photography  of  the  corona.  Unquestionably,  there  is  much 
to  be  said  in  favor  of  securing  photographs  of  great  scale  by 
tower  telescopes. 

The  present  writer  would  like  to  raise  another  question, 
namely,  what  scientific  use  is  being  made  (other  than  that  of 
providing  important  historical  documents)  of  the  long  series 
of  large-scale  coronal  photographs  accumulated  by  Lick, 


THE  ECLIPSES  OF  1932  AND  1934  497 

Greenwich  and  other  observatories?  At  the  same  time  we 
might  ask,  what  scientific  uses  are  being  made  of  photographs 
to  the  hundreds  of  thousands,  of  solar  details  like  sun  spots 
and  prominences  accumulated  through  long  series  of  years  by 
many  observatories?  The  answers  to  these  questions  and  to 
many  others  of  similar  nature  dealing  with  solar  problems  can 
furnish  topics  of  valuable  discussions  for  many  future  sessions 
of  the  Solar  Physics  Commission  of  the  International  Astro- 
nomical Union. 

Fortunately,  photographs  of  all  scales,  except  the  small,  are 
of  great  value  for  determining  the  shape  of  the  corona  by  the 
method  devised  by  Ludendorff  and  described  on  page  401. 
Recently  two  articles  on  this  subject  have  appeared,  one x  by 
Ludendorff,  the  other  2  by  Bergstrand.  On  account  of  a  slight 
error  in  Ludendorff's  published  formula,  the  values  given  on 
page  402  must  be  altered  in  two  cases.  For  the  1922  eclipse, 
the  corrected  value  for  the  quantity  a  is  +  o.oi,  and  for 
by  +  0.22;  and  for  the  1930  eclipse  the  values  should  read 
+  0.03  and  +  o. 2 2,  respectively.  For  the  1932  eclipse,  Miss 
Williams  derived  values  for  the  ellipticity  from  measures  of 
photographs,  and  found  a  was  +  0.05  and  b,  +  0.22,  the 
value  for  the  spot  numbers  being  6.  For  the  same  eclipse  Lu- 
dendorff, from  fewer  photographs,  derived  values  for  a  and  b 
of  +  °-°6  and  +  o-M?  respectively.  Measures  of  1932  coro- 
nal photographs,  however,  showed  quite  conclusively  that 
values  of  a  and  b  from  photographs  by  one  camera  were  quite 
consistent  among  themselves  but  differed  systematically  from 
values  from  other  photographs.  Details  cannot  be  given  here 
but  will  appear  in  the  forthcoming  volume  of  Handbuch  der 
Astrophysik. 

For  the  eclipse  of  February  14,  1934,  the  shape  of  the  co- 
rona was  determined  from  an  excellent  photograph  by  Satome 
reproduced  facing  page  500  and  from  isophotes  furnished  in 
advance  of  publication  by  Cohn  from  photographs  polarized 
in  directions  at  right  angles  to  each  other.  The  ellipticity  of 
the  corona  in  one  of  Cohn's  photographs  differs  radically  from 
the  ellipticity  of  the  companion  photograph  polarized  in  a 

1  Sitz.  Preuss.  Akad.,  Phys.  Math.  Kl.,  16,  1934. 

2  Arkiv.  Mat.  Astr.  och  Physik.,  25 A,  No.  4,  1934. 


498  THE  ECLIPSES  OF  1932  AND  1934 

plane  perpendicular,  thus  showing  the  same  effect  displayed 
by  the  pair  of  photographs  taken  at  the  1908  eclipse,  repro- 
duced facing  page  388.  The  shape  of  the  1934  corona  was  de- 
rived by  following  the  Ludendorff  method  and  determining 
separately  the  ellipticities  of  the  photographs  polarized  in  the 
two  directions,  and  then  taking  the  mean,  the  results  being 
confirmed  by  measures  from  the  Satome  photograph.  For 
this  eclipse,  a  was  found  to  be  +  0.08  and  b,  +  0.14,  making 
a  +  b  equal  to  +0.22.  The  value  of  "  spot  numbers  "  (page 
402)  is  8,  the  spots  being  on  the  increase.  The  eclipse  took 
place  0.2  years  after  the  sun-spot  minimum. 

As  shown  on  page  402,  values  of  a  differ  little  from  eclipse 
to  eclipse  while  there  is  quite  a  range  in  the  values  of  b.  Lu- 
dendorff has  adopted  a  +  b,  or  the  ellipticity  of  the  corona  at 
a  distance  of  one  radius  from  the  edge  of  the  sun,  or  two  radii 
from  the  sun's  center,  as  the  best  measure  for  ascertaining  the 
shape  of  the  corona.  To  derive  the  relative  activity  of  the  sun 
in  the  i  i-year  cycle  there  is  a  wealth  of  published  material  on 
both  spots  and  prominences.  For  the  sun  spots,  it  has  seemed 
wisest  always  to  derive  a  mean  from  values  spread  over  a 
whole  synodic  revolution  of  twenty-seven  days,  that  is,  a 
mean  of  thirteen  days  both  before  and  after  and  also  the  day 
in  question.  The  values  for  "  spot  numbers  "  on  page  402, 
derived  from  information  published  at  Zurich,  correspond  to 
average  values  referred  to  the  day  of  the  eclipse.  In  a  similar 
way,  Ludendorff  derives  values  for  the  spot  numbers  for  one, 
two  and  three  months,  respectively,  before  the  eclipse,  for  the 
average  annual  value,  and  also  for  the  phase  in  the  sun-spot 
cycle  depending  on  the  times  of  sun-spot  maxima  and  minima. 
In  a  similar  way  (loc.  cit.}  corresponding  values  are  found 
from  prominences.  With  these  values  in  hand,  Ludendorff 
plots  as  abscissae  various  sun-spot  (or  prominence)  numbers, 
and  as  ordinates  the  values  (a  +  #)  representing  the  shapes 
of  the  various  coronas.  An  attempt  to  pass  a  smooth  curve 
through  any  one  of  the  individual  graphs  shows  that  the  1918 
eclipse  falls  far  distant  from  any  curve.  With  a  total  of  eight- 
een eclipses  starting  in  1893,  measured  and  plotted  in  a  uni- 
form manner  (leaving  out  of  consideration  the  discordant 
eclipse  of  1918)  it  is  difficult  to  say  which  of  the  dozen  curves 


THE  ECLIPSES  OF  1932  AND  1934  499 

makes  the  best  fit  or  shows  the  closest  correlation  between  the 
shape  of  the  corona  and  solar  activity. 

Bergstrand  uses  LudendorfP s  measures  of  a  and  b  and  de- 
rives for  each  eclipse  a  coronal  number  p  and  an  ellipticity  JEi. 
These  are  plotted  against  the  different  numbers  found  from 
spots  and  prominences,  and  smooth  curves  are  again  drawn. 
It  is  true  that  these  plotted  points  lie  closer  to  the  curves  (the 
1918  eclipse  is  still  discordant)  than  do  the  similar  points 
plotted  against  (a  +  6)  by  Ludendorff.  However,  the  num- 
bers p  and  £1  used  by  Bergstrand  are  not  very  sensitive  to 
changes  in  the  shape  of  the  corona  as  is  shown  by  the  fact  that 
throughout  the  period  from  one  year  after  spot  maximum 
until  one  year  after  spot  minimum  there  is  very  little  varia- 
tion in  values  of  either  of  the  two  Bergstrand  quantities. 

For  the  eclipse  of  1900,  the  value  of  (a  +  6)  which  is  equal 
+  0.30  is  the  greatest  of  any  of  the  eighteen  eclipses  consid- 
ered, or  the  eclipse  in  the  last  year  of  the  i9th  century  showed 
the  most  pronounced  "  minimum-type  "  corona  with  long 
equatorial  streamers  and  pronounced  solar  brushes.  The 
average  value  of  the  probable  error  of  a  +  b  is  ^t  .03.  Those 
familiar  with  the  theory  of  least  squares  realize  that  indi- 
vidual values  may  readily  have  discordances  of  two  or  three 
times  the  probable  error.  In  addition  to  the  accidental  errors 
(represented  by  the  probable  error)  there  may  be  systematic 
errors  as  was  shown  by  the  1932  eclipse.  If  there  is  an  active 
sun  spot  or  prominence  at  the  sun's  edge  there  may  be  a  long 
streamer  in  the  corona,  as  was  shown  (page  404)  by  the  1930 
eclipse,  which  might  cause  changes  in  the  measured  shape  of 
the  corona. 

Taken  all  in  all,  it  is  too  much  to  expect  that  the  rough 
method  of  deriving  the  shape  of  the  corona  could  possibly  give 
points  that  would  lie  close  to  a  smooth  curve.  With  a  total  of 
only  eighteen  points,  one  of  them  (1918)  very  discordant,  it 
is  not  worth  while  to  waste  time  to  find  which  of  the  many 
curves  gives  the  closest  correlation  between  coronal  shape 
and  sun-spot  or  prominence  data. 

The  accompanying  curve  gives  as  ordinates  the  values  of 
100  (a  +  6)  and  as  abscissae  the  values  of  the  phases  in  the 
sun-spot  cycle,  O  in  the  middle  of  the  diagram  representing 


500 


THE  ECLIPSES  OF  1932  AND  1934 


spot  minimum.  The  —  signs  signify  that  the  spots  are  de- 
creasing from  maximum,  at  the  left  of  the  diagram,  towards 
minimum,  and  the  -f-  sign  that  the  spots  are  increasing  in 
numbers  after  minimum  has  passed.  The  curve  was  drawn  to 
fit  as  well  as  possible  all  of  the  observations  including  the  co- 
rona of  1918.  It  is  unfortunate,  however,  that  the  six  points 
on  the  left  side  of  the  diagram  representing  the  shapes  of  the 
corona  in  the  years  following  maximum  of  sun  spots  are  on  the 
whole  quite  discordant  while  the  other  twelve  eclipses  to 
the  right  are  more  consistent  and  hug  the  curve  much  more 
closely.  Two  eclipses  of  the  six,  those  of  1896  and  1898,  tell 
quite  different  stories  in  that  the  corona  of  the  earlier  year 
was  more  elliptical  in  shape  than  that  of  two  years  later.  The 


I I I I I 


only  method  of  determining  a  more  exact  connection  between 
the  shape  of  the  corona  and  solar  activity  will  be  to  acquire 
more  observations,  particularly  on  the  left  side  of  the  curve, 
which  may  be  done  only  by  observing  future  eclipses  in  the 
nineteen-forties. 

Various  other  graphs  were  made  connecting  coronal  shapes 
with  spot  or  prominence  numbers,  but  all  of  the  curves  with- 
out exception,  no  matter  what  spot  or  prominence  numbers 
are  plotted,  tell  the  same  story  exhibited  in  the  curve  plotted 
to  sun-spot  phase.  The  most  pronounced  minimum  type  of 
corona  does  not  take  place  exactly  at  sun-spot  minimum. 
From  the  curve  it  is  seen  that  two  and  a  half  years  before  spot 
minimum  the  corona  is  quite  as  elongated  as  it  is  at  the  time 
of  minimum  of  spots,  or  phase  O  in  the  curve.  The  most  elon- 
gated corona  is  found  one  and  a  quarter  years  before  mini- 
mum of  spots,  and  likewise  the  corona  closest  in  shape  to  a 


§ 


I 

w 

o 

it 


O    o> 

I! 


1 


COHN  AND  JAPANESE  ASSISTANTS  (ABOVE)  AND  JOHNSON  (BELOW) 
AT  THE  1934  ECLIPSE 


THE  ECLIPSES  OF  1932  AND  1934  501 

circle  takes  place  one  and  a  quarter  years  before  spot  maxi- 
mum. 

The  recent  minimum  occurred  at  the  year  1933.9  and  hence 
the  1932  eclipse  took  place  1.2  years  before  spot  minimum, 
almost  at  the  highest  part  of  the  curve.  The  1 934  eclipse  took 
place  very  close  (0.2  years  after)  to  spot  minimum.  With 
a  +  b  equal  +  0.22  it  is  evident  that  the  corona  had  lost  its 
minimum  type  characteristics.  The  1936  corona  taking  place 
24  years  after  spot  minimum  should  be  intermediate  between 
minimum  and  maximum  types.  It  and  the  1937  eclipse  will 
fill  gaps  in  the  curves  connecting  the  shape  of  the  corona  and 
sun-spot  numbers. 

When  arranged  according  to  phase  in  the  sun-spot  cycle, 
the  eighteen  eclipses  fall  naturally  into  six  groups  of  three 
eclipses  each  as  given  in  the  table.  The  mean  values  for  the 
phases  are  given  below.  The  three  eclipses,  1900,  1922  and 

THE  SHAPE  OF  THE  CORONA  AND  SUN-SPOT  PHASE 


1908  -  .78  '  1896 
1918  -  .87  ,  1898 
1929  —  .83  i  1930 

-  -67 
-  -47 
-•57 

1900 
1922 
1932 

—  .17  i  1901  —  .04  1914  +  .25  !  1893  4-  .82 
-  .15  |  1923  -f  .02  i  1925  -f  .31  1905  +  -85 
—  .24  ,  1934  -h  .04  ^  1926  -f  .50  1927  -h  -81 

Mean  —  .83  j 

-  -57 

-  .19  I      -f  .01       +  .35      +  .83 

1932,  each  of  which  showed  so  beautifully  the  so-called 
"  minimum  type  "  of  corona  had  a  mean  phase  of  —  .19, 
while  the  "  maximum  type  "  of  corona  of  the  three  eclipses 
1893,  I9°S  and  J927  had  +  .83  for  the  average  mean  phase. 
From  the  table  it  is  evident  that  eclipses  should  be  divided 
into  maximum  and  minimum  types  and  into  two  types  (rather 
than  one  type)  intermediate  between  maximum  and  minimum 
and  between  minimum  and  maximum. 

The  astronomers  of  the  world  have  been  greatly  thrilled  by 
the  skillful  work  of  Lyot  in  photographing  the  corona  without 
an  eclipse.  A  later  publication  *  than  those  alluded  to  on  page 
419  gives  the  details  of  his  valuable  work.  The  great  triumph 
of  Lyot  resulted  from  his  ingenious  arrangement  of  his  tele- 
scope to  reduce  to  a  minimum  the  amount  of  instrumental 
scattered  light.  A  mountain  observatory  on  the  Pic  du  Midi 
(9100  feet)  permitted  photographs  at  times  when  the  sky  was 

1  Zs.  /.  Astroph.,  5,  73,  1932. 


502  THE  ECLIPSES  OF  1932  AND  1934 

exceptionally  clear,  or  when  the  intensity  of  scattered  sky 
light  was  less  than  five-millionths  that  of  the  sun.  By  com- 
bining the  coronagraph  with  a  spectrograph  it  was  possible  to 
determine  the  wave-lengths  of  both  the  green  and  red  coronal 
lines.  A  concave  grating  gave  a  dispersion  of  1.2  A  per  mm 
and  a  prism  arranged  Littrow  type  gave  a  dispersion  in  the 
green  of  5  A  per  mm.  This  large  scale  of  wave-lengths  per- 
mitted Lyot  to  measure  the  width  of  green  line  with  the  sur- 
prising result  that  the  total  width  is  1.2  angstroms. 

"  As  a  consequence  of  having  36  spectra  of  large  dispersion, 
it  was  naturally  assumed  that  the  wave-length  of  the  green 
line  had  an  accuracy  greater  than  had  resulted  from  eclipse 
spectra.  Similar  conclusions  were  drawn  for  the  accuracy  of 
the  red  line.  At  the  1930  eclipse,  Mitchell  had  eight  spectra, 
on  all  but  two  of  which  measures  were  made  of  the  wave- 
length of  the  green  line  on  both  E  and  W  limbs  of  the  sun. 
The  wave-length  is  therefore  the  result  of  fourteen  inde- 
pendent measures  with  a  dispersion  of  10.9  A  per  mm.  For 
the  red  line  there  were  eight  separate  measures  on  six  spectra. 
The  wave-lengths  and  probable  errors  are  given,  together 
with  similar  values  by  Lyot. 

Lyot  Mitchell 

5302.85  ±  .03  5302.91  ±  .02 

6374.75  ±  .15  6374.28  ±  .03 

It  is  evident  that  Lyot's  claim  of  greater  accuracy  and  a 
similar  claim  in  Monthly  Notices,  93,  2  78,  1933,  of "  an  accu- 
racy far  greater  than  that  obtained  hitherto  "  cannot  be  sub- 
stantiated as  is  shown  by  the  sizes  of  the  probable  errors. 

By  means  of  a  Lyot  telescope  constructed x  at  Mount  Wil- 
son, Pettit  and  Slocum  secured  direct  photographs  of  promi- 
nences. 

No  coronal  wave-lengths  have  yet  (1935)  been  published 
for  the  1932  and  1934  eclipses.  After  many  unsuccessful  at- 
tempts at  previous  eclipses  (page  385),  a  photograph  of  the 
green  line  has  finally  been  secured  by  the  interferometer.  At 
the  1932  eclipse  this  method  was  tried  by  the  members  of 
three  different  expeditions.  Farthest  west,  Carroll  had  a 


THE  ECLIPSES  OF  1932  AND  1934  503 

beautifully  designed  instrument,  but  again  he  had  bad  luck 
with  the  weather,  and  his  record  now  is  four  cloudy  eclipses  in 
four  attempts.  At  Fryeburg  there  were  two  interferometers, 
at  the  University  of  Michigan  and  Lick  expeditions.  By 
means  of  a  double  slit  on  the  Lick1  interferometer,  an  at- 
tempt was  made  to  photograph  fringes  both  E  and  W  of  the 
sun.  At  one  limb  there  was  no  trace  of  the  green  line  but  on 
the  other  it  was  strongly  photographed.  Although  fringes 
were  very  distinct  in  the  comparison  lines,  no  evidence  of 
fringes  were  visible  in  the  coronal  line  and  measures  with  a 
microphotometer  revealed  a  mere  suggestion  of  the  correct 
periodicity.  Lyot's  photographs  explain  the  absence  of 
fringes,  namely,  that  the  surprising  width  of  the  green  coronal 
line  of  one  angstrom  would  permit  distinct  fringes  only  with  a 
very  small  separation  of  the  interferometer  plates,  which  un- 
fortunately would  entail  a  very  low  accuracy  in  the  wave- 
lengths. Hence  in  the  moment  of  triumph  of  securing  the 
first  photographs  of  the  coronal  line  with  the  interferometer 
there  is  the  realization  that  further  attempts  must  be  aban- 
doned. This  must  be  a  keen  disappointment  to  those  astrono- 
mers who  have  spent  so  much  time  and  energy  and  money  in 
perfecting  the  interferometer  to  secure  accurate  wave-lengths 
of  the  green  line.  The  disappointment,  however,  is  part  of  the 
game  of  being  a  scientist. 

At  the  1930  eclipse,  Mitchell's  photographs  (page  377) 
showed  both  the  green  and  red  coronal  lines  in  the  short  ex- 
posures given  for  the  flash  spectrum.  It  is  therefore  not  sur- 
prising to  find  that  with  the  Mt.  Wilson  tower  telescope  Bab- 
cock  has  obtained  the  green  coronal  line  on  photographs  of  the 
chromosphere  without  an  eclipse.  On  the  original  photo- 
graphs (which  the  writer  has  seen)  the  5303  line  is  so  wide 
and  hazy  that  an  accurate  wave-length  is  impossible  in  spite 
of  the  large  scale  of  the  photographs.  Adams  and  Joy2  at 
Mt.  Wilson  make  the  surprising  observation  that  five  coronal 
lines,  3987,  4086,  4231,  5303  and  6374,  were  present  at  one 
stage  of  the  outburst  of  the  Nova-like  RS  Ophiuchi. 

The  best  wave-length  for  the  green  line  is  5302.90  ±  .02 

1  Publ  A.  S.  P.,  44>  358,  1932. 

2  Ibid.,  45,  301,  1933. 


504  THE  ECLIPSES  OF  1932  AND  1934 

which  is  derived  by  combining,  with  equal  weights  given  to 
each,  the  Lick-Mt.  Wilson  value  of  5302.93  (page  374)  and 
the  Lyot  and  Mitchell  values.  For  the  red  line,  perhaps  the 
best  method  will  be  to  combine  the  Lyot  and  Mitchell  values 
with  weights  depending  on  the  probable  errors.  The  derived 
value  is  6374.30  =fc  .03,  in  close  agreement  with  the  labora- 
tory value  of  neutral  oxygen  at  6374.29  A. 

The  great  difference  in  the  wave-lengths  of  the  two  best 
values  for  the  red  line  amounting  to  nearly  half  an  angstrom 
shows  the  need  of  new  determinations.  Until  more  accurate 
values  are  obtained  for  the  whole  coronal  spectrum,  it  seems 
a  waste  of  time  to  devote  much  energy  to  deciphering  the  rid- 
dle of  coronium.  Since  the  date  of  the  attempts  recorded  on 
page  382,  other  individuals  have  made  widely  published 
claims.  The  consensus  of  well-informed  opinion  l  seems  to  be 
that  although  it  is  quite  probable  that  neutral  oxygen  may 
eventually  be  found  to  be  one  of  the  chief  constituents  of 
coronium,  at  present  that  fact  is  far  from  being  proved. 

In  the  first  burst  of  enthusiasm  over  Lyot's  remarkable 
work,  it  was  thought  by  many  that  another  eclipse  problem, 
that  of  coronal  wave-lengths,  could  be  attacked  when  there 
was  no  eclipse.  Success  on  an  isolated  mountain  peak  like 
Pic  du  Midi  requires  remarkable  transparency  of  sky  and 
conditions  of  seeing  which  come  but  seldom.  In  cost  such  an 
expedition  is  commensurate  with  that  of  an  eclipse  expedition. 
Through  perseverance,  Mt.  Wilson  may  finally  succeed  in  de- 
riving accurate  wave-lengths  for  the  green  and  red  coronal 
lines.  Of  equal  importance,  however,  are  better  wave-lengths 
for  the  balance  of  the  coronal  lines.  At  present  there  seems  to 
be  no  method  of  securing  this  information  so  badly  needed  for 
finding  the  origin  of  coronium,  than  by  the  use  of  well- 
designed  spectrographs  during  the  total  phase  of  an  eclipse. 
Photographs  which  give  accurate  wave-lengths  of  the  fainter 
lines  of  the  corona  will  of  necessity  give  a  greater  accuracy  for 
the  brighter  green  and  red  lines  so  that  eventually  we  may  be 
able  to  ascertain  something  about  the  rotation  of  the  corona, 
a  subject  concerning  which  we  know  practically  nothing. 

At  the  1932  eclipse,  photo-electric  cells  were  used  at  many 

1  R.  Frerichs,  Die  Naturwiss.,  48,  849,  1933. 


THE  ECLIPSES  OF  1932  AND  1934  505 

expeditions  in  attempts  to  measure  the  light  of  the  corona. 
At  Magog,  we  had  two  separate  set-ups  of  Weston  Illumina- 
tion Meters,  and  we  were  fortunate  in  that  E.  F.  Weston  was 
there  in  person.  An  automatic  record  was  made  of  the  sky 
illumination  near  the  North  Pole  starting  before  daylight  and 
continuing  throughout  the  whole  day  until  after  dark.  An- 
other set  of  sixteen  cells  was  directed  towards  the  sun  and  the 
field  of  illumination  was  confined  to  a  circle  of  8°  in  diameter 
with  the  eclipsed  sun  in  the  center.  Although  clouds  pre- 
vailed during  totality,  the  records  were  sent  to  Dr.  M.  Min- 
naert  who  in  part  reports,  "  It  is  easy  to  show  that  during  the 
partial  phases  the  illumination  of  the  sky  surrounding  the  sun 
was  chiefly  due  to  direct  solar  radiation,  but  during  totality 
the  illumination  was  due  mainly  to  reflection  of  the  light  from 
points  outside  the  path  of  totality. "  Although  the  clouds  at 
Magog  were  denser  than  they  were  (for  instance)  at  Frye- 
burg,  one  seems  compelled  to  raise  the  question  whether  the 
Fraunhofer  lines  shown  in  the  spectra  of  the  corona  taken  by 
Moore  at  this  eclipse  are  the  result  of  coronal  radiation  or 
whether  they  are  not  in  part  caused  by  sunlight  reflected  by 
the  thin  clouds  which  were  present  throughout  totality.  On 
the  photographs,  the  effects  would  be  greatest  at  the  begin- 
ning and  the  ending  of  totality. 

In  this  connection  it  is  well  to  again  raise  the  question,  al- 
ready raised  on  page  397,  as  to  what  information  we  have 
from  previous  eclipses  regarding  the  total  light  of  the  corona. 
Regarding  the  measures  of  the  illumination  of  corona  plus  sky 
by  means  of  illuminometers,  one  competent  photometric  au- 
thority states,1  "  There  can  be  no  objection  to  anyone  meas- 
uring the  normal  illumination  on  a  completely  exposed  test 
plate  at  the  time  of  totality  if  he  is  convinced  of  the  utility 
of  so  doing.  Trouble,  however,  may  ensue  when  the  results  of 
such  measurement  are  seriously  put  forward  as  indicative  of 
the  amount  of  light  which  the  corona  is  giving." 

The  light  of  the  corona  which  has  been  seen  to  extend  to  the 
enormous  distance  of  ten  million  miles  from  the  sun's  surface 
has  three  separate  spectra  (page  382):  (i)  the  bright  line 
spectrum  of  "  coronium,"  (2)  a  continuous  spectrum  in  the 

1  Jour.  Opt.  Socy.  America,  23,  234,  1933. 


506  THE  ECLIPSES  OF  1932  AND  1934 

brighter  inner  corona,  and  (3)  a  Fraunhofer  spectrum  in  the 
middle  and  outer  corona.  Recently  very  substantial  progress 
has  been  made  in  investigating  the  continuous  spectrum  and 
also  the  dark  line  spectrum,  and  thereby  valuable  informa- 
tion concerning  the  physical  origin  of  the  corona  has  been 
given. 

At  the  eclipse  of  1929,  with  three  different  spectrographs, 
Grotrian  l  secured  spectra  of  the  corona  which  were  compared 
with  the  solar  spectrum  taken  with  the  same  instruments  but 
diminished  in  intensity  by  known  amounts.  Measurements 
made  with  the  microphotometer  give  the  following  interesting 
results,  (i)  The  position  of  the  energy  maximum  in  the  co- 
rona is  independent  of  the  height  in  the  corona  and  agrees 
exactly  with  the  energy  maximum  of  the  solar  spectrum.  (2) 
Within  errors  of  measurement,  the  distribution  of  energy  in 
the  coronal  spectrum  is  independent  of  height  in  the  corona 
and  is  identical  with  the  intensity  of  radiation  in  the  solar 
spectrum.  These  measures  confirm  the  conclusions  of  Luden- 
dorff l  from  the  1923  eclipse.  After  many  long  years  of  un- 
certainty, at  last  we  seem  to  know  for  certain  that  the  color  of 
the  corona  is  identical  with  that  of  the  sun,  or,  in  other  words, 
the  corona  appears  to  shine  from  scattered  sunlight. 

In  the  remarkably  clear  sky  at  the  1922  eclipse  (page  383), 
Moore  seemed  to  prove  conclusively  that  the  Fraunhofer  lines 
in  the  middle  and  outer  corona  have  their  origin  in  the  corona 
itself  and  are  not  caused  by  sunlight  scattered  by  the  earth's 
atmosphere.  By  means  of  a  microphotometer,  Grotrian  3 
measured  the  widths  of  the  stronger  Fraunhofer  lines  in  the 
coronal  spectrum  taken  with  one  prism  at  the  1923  eclipse. 
These  widths  were  compared  with  those  of  the  solar  spectrum 
taken  by  the  same  spectrograph  and  weakened  by  known 
amounts.  In  every  case  the  widths  of  lines  in  the  coronal 
spectrum  were  compared  with  the  widths  in  both  stronger  and 
weaker  solar  spectra.  All  observers  heretofore  had  thought 
that  the  lines  in  the  coronal  spectrum  were  both  wider  and  less 
distinct  than  similar  lines  in  the  sun.  On  the  contrary,  Gro- 
trian's  measures  show  that  although  the  dark  lines  in  the  co- 

1  Zs.  fur  Astrophysik,  3,  199,  1931. 

2  Sitz.  d.  preuss.  Akad.  d.  Wiss.,  5,  83,  1925. 

3  Zs.  jur  Astrophysik,  8,  124,  1934. 


THE  ECLIPSES  OF  1932  AND  1934  507 

ronal  spectrum  certainly  appear  less  distinct  than  in  the  solar 
spectrum,  and  moreover  that  the  dark  lines  are  much  more 
prominent  in  the  outer  corona  than  in  the  inner,  nevertheless, 
the  widths  of  the  coronal  lines  are  the  same  at  all  distances  out 
from  the  sun's  surface,  and  within  errors  of  measurement,  are 
identical  in  width  with  the  corresponding  lines  in  the  solar 
spectrum.  From  similar  measures  carried  out  on  the  1922 
and  1932  eclipse  spectra,  Moore  l  confirms  Grotrian's  meas- 
ures of  the  1923  eclipse  spectra. 

Leaving  out  of  consideration  for  the  time  being  the  bright 
line  spectrum  of  coronium,  which  remains  a  perplexing 
eclipse  problem,  it  is  evident  that  the  coronal  spectrum 
consists  of  two  spectra  overlapping  each  other,  the  con- 
tinuous and  the  Fraunhofer.  The  1923  spectrum  showed 
Fraunhofer  lines  superposed  on  the  dark  moon.  On  the  as- 
sumption that  these  were  caused  by  scattered  coronal  light, 
either  in  the  earth's  atmosphere  or  in  the  spectrograph,  Gro- 
trian  was  able  to  apply  corrections  to  the  measured  intensities 
of  the  dark  lines  in  the  coronal  spectrum.  These  corrected 
intensities  show  that  near  the  sun's  edge  the  continuous  spec- 
trum is  considerably  stronger  than  the  Fraunhofer  spectrum 
so  that  no  dark  lines  are  visible,  due  to  lack  of  contrast, 
though  no  doubt  they  are  there.  At  increasing  distances  from 
the  sun,  the  intensity  of  the  continuous  spectrum  falls  off 
more  rapidly  than  the  dark  line  spectrum.  At  a  distance  of 
about  one  solar  radius,  the  intensities  of  the  two  spectra  are 
equal  and  the  Fraunhofer  lines  then  become  the  more  readily 
visible  in  the  outer  corona.  From  what  has  been  said  on  page 
505,  it  is  quite  unsafe  to  assume,  especially  under  the  con- 
ditions experienced  at  the  1923  eclipse,  that  the  dark  lines 
of  the  spectrum  superposed  on  the  face  of  the  moon  were 
actually  caused  by  scattered  coronal  light  rather  than  by 
sunlight  scattered  by  the  earth's  atmosphere  especially  at 
times  near  the  beginning  and  ending  of  totality.  This,  how- 
ever, is  a  mere  detail  that  affects  the  relative  intensities  of  the 
continuous  and  Fraunhofer  spectra  at  different  distances  from 
the  sun's  edge,  but  it  in  no  way  detracts  from  the  remarkably 
fine  work  of  Grotrian. 

1  Publ.  A.  S.  P.,  46,  298,  1934. 


508  THE  ECLIPSES  OF  1932  AND  1934 

According  to  Rosseland,1  the  corona  has  "  stimulated  specu- 
lation to  the  breaking  point,  it  being  even  suggested  that  there 
we  witness  our  recognized  physical  laws  set  at  naught  by 
nature  herself."  As  a  result  of  many  theories  of  the  corona 
(page  415),  it  is  now  generally  recognized  that  the  electron 
must  play  an  important  role  in  explaining  the  radiation  of  the 
corona.  If  the  continuous  spectrum  is  caused  by  free  elec- 
trons which  scatter  the  solar  radiation,  Rosseland  finds  that 
at  a  temperature  of  4000°  K,  the  average  thermal  velocity 
of  the  electron  would  cause  a  Doppler  effect  of  the  order  of 
ten  angstroms.  As  a  result,  all  of  the  dark  Fraunhofer  lines 
will  be  wiped  out?  with  the  exception  of  the  broad  wings  of 
the  H  and  K  lines. 

Electron  scattering  therefore,  furnishes  an  adequate  ex- 
planation of  the  absence  of  dark  lines  in  the  spectrum  of 
the  inner  corona  and  also  is  in  conformity  with  the  fact  that 
there  is  no  difference  in  color  of  the  corona  from  the  sun.  But 
how  explain  the  dark  lines  in  the  outer  parts  of  the  corona? 
If  the  electron  is  to  be  the  agent,  then  it  is  necessary  to  find 
some  process  whereby  the  random  velocities  of  the  electrons 
may  be  slowed  down  to  such  an  extent  that  the  Doppler  ef- 
fects will  not  obliterate  the  Fraunhofer  lines.  It  is  scarcely 
possible  to  assume  a  temperature  in  the  middle  corona  less 
than  2000°  K  which  would  cause  a  Doppler  effect  of  about 
seven  angstroms.  We  might,  if  we  please,  assume  some  un- 
known (as  yet)  influence  from  positrons  or  that  the  sun's 
magnetic  field  or  some  other  unknown  source  might  gradu- 
ally slow  up  the  electron  velocities  at  distances  of  10'  from  the 
sun's  surface.  If,  indeed,  there  were  a  gradual  slowing  up  of 
the  electrons,  this  would  cause  the  Fraunhofer  lines  to  be 
broader  in  the  middle  corona  and  sharper  in  the  outer  corona 
—  which  is  not  in  accord  with  the  facts. 

Under  the  assumption  that  the  light  of  the  corona  is  caused 
by  the  superposition  of  a  continuous  spectrum  over  a  dark 
line  spectrum,  it  is  necessary  to  conclude  that  electrons  are 
present  everywhere  throughout  the  corona.  As  atoms  and 
ions  are  present  in  the  inner  corona  causing  the  bright  line 
spectrum  of  coronium,  it  has  been  thought  by  many  that 

1  University  Observatory,  Oslo,  Publ.,  No.  5,  1933. 


THE  ECLIPSES  OF  1932  AND  1934  509 

ionized  atoms  must  be  the  cause  of  the  dark  lines  of  the 
coronal  spectrum.  We  do  not  see  how  it  would  be  possible 
(page  417)  for  atoms  to  exist  in  the  outer  corona  in  appre- 
ciable numbers  and  moreover  they  would  cause  a  Rayleigh 
scattering  and  a  distribution  of  intensity  in  the  corona  dif- 
ferent from  that  in  the  sun  —  which  again  is  not  in  accord 
with  the  facts. 

After  discussing  many  possibilities,  Grotrian  is  forced  to 
the  conclusion  that  the  sun's  light  is  scattered  by  solid  par- 
ticles with  diameters  greater  than  three  times  the  wave- 
length of  light  which  would  cause  a  scattering  of  sunlight 
without  change  in  color.  Close  to  the  sun  (page  411),  solid 
particles  would  be  immediately  vaporized  but  at  greater  dis- 
tances there  would  be  an  increasing  number  of  solid  particles; 
in  fact,  in  the  zodiacal  light  we  have  independent  evidence  of 
the  existence  of  such  material  in  interplanetary  space.  By 
making  plausible  assumptions,  Grotrian  shows  that  the  sur- 
face brightness  of  the  outer  corona  bears  to  the  brightness  of 
the  zodiacal  light  a  ratio  of  about  the  right  order  of  magni- 
tude. This  marks  a  partial  return  to  the  theory  of  Arrhenius 
(page  410).  Within  one  radius  of  the  sun's  surface  we  must 
regard  the  electron  as  the  scattering  mechanism  but  farther 
out  the  interplanetary  dust  does  most  of  the  scattering. 

The  eclipses  of  1932  and  1934  have  cleared  up  many  ques- 
tions regarding  the  polarization  of  light  surrounding  the  sun 
at  the  time  of  totality.  Polarization  measures  must  be  the 
result  of  a  combination  of  the  partly  polarized  light  from  the 
corona  and  the  unpolarized  light  in  the  earth's  atmosphere. 
The  measures  of  Young  (page  388)  showed  that  the  per- 
centage of  polarization  in  photographic  light  was  about 
three  times  greater  than  in  visual  light,  which  was  explained 
by  the  assumption  that  the  coronal  radiation  is  Rayleigh 
scattering  from  atoms  and  ions.  If  however,  the  corona  is 
due  to  scattering  by  the  electron,  then  the  amount  of  polari- 
zation must  be  the  same  in  photographic  as  in  visual  light. 
Dufay  and  Grouiller  l  show  that  at  the  eclipse  of  1932  the 
amount  of  polarization  is  independent  of  wave-length  and  is 
a  maximum  of  26  per  cent  at  10'  from  the  sun's  edge.  At 

1  C.  R.,  196,  1574,  1933. 


510  THE  ECLIPSES  OF  1932  AND  1934 

the  eclipse  of  1934,*  from  visual  measurements,  Johnson  finds 
a  maximum  of  28  per  cent  polarization  at  8'. 5  from  the  sun's 
limb,  a  result  in  good  agreement  with  the  1932  eclipse. 

Fortunately  many  of  the  difficulties  that  retarded  progress 
in  our  understanding  of  the  corona  have  been  cleared  away. 
The  assumption  that  the  corona  consists  essentially  of  free 
electrons  permits  us  to  apply  the  theory  of  Minnaert.2  When 
knowledge  progresses  still  farther  and  we  learn  more  about 
the  physical  laws  that  govern  matter  under  solar  conditions, 
we  may  be  able  to  find  an  explanation  for  the  dark  lines  in 
the  coronal  spectrum  that  is  simpler  than  the  assumption  that 
they  are  caused  by  the  scattering  of  the  sun's  light  by  in- 
terplanetary dust. 

For  future  progress  we  need  more  and  better  spectra  of 
the  dark  lines  in  the  coronal  spectrum  taken  under  clear  skies 
devoid  of  water  vapor  in  order  to  be  certain  that  the  Fraun- 
hofer  lines  are  coronal  in  origin  and  do  not  come  from  sun- 
light scattered  in  the  earth's  atmosphere.  We  need  more 
and  better  spectra  of  large  dispersion  in  order  to  get  more 
accurate  wave-lengths  of  the  bright  line  spectrum  by  which 
to  find  the  origin  of  coronium  and  the  rotation  of  the  corona. 
In  spite  of  the  splendid  achievement  of  Lyot,  new  information 
regarding  the  corona  will  come  entirely  from  eclipse  expedi- 
tions, and  observations  will  be  accumulated  at  the  average 
rate  of  one  minute  of  time  per  year. 

Eclipse  expeditions  are  still  needed  in  the  future  to  secure 
additional  information  about  the  chromosphere.  In  pages 
294-98,  the  author  gave  recommendations  for  future  work. 
He  would  like  to  clarify  his  meaning  by  urgently  recommend- 
ing the  use  of  both  fixed  and  moving  plates  for  securing  photo- 
graphs of  the  flash  spectrum.  In  addition  he  would  like  to 
quote  the  opinions  of  the  Greenwich  eclipse  observers.3 
"  The  great  need  for  the  future  appears  to  be  accurate  pho- 
tometric intensities  of  the  lines,  especially  of  multiplets,  and 
these  can  only  be  determined  when  a  comparison  spectrum 
can  be  secured  with  the  same  instrument.  The  comparison 

1  Publ.  A.  S.  P.,  46,  226,  1934. 

2  Zs.  fur  Astrophysik,  i,  209,  1930. 
8  Monthly  Notices,  pj,  i,  1932. 


THE  ECLIPSES  OF  1932  AND  1934  511 

spectrum  must  be  obtained  from  the  uneclipsed  sun  or  an 
artificial  source,  and  for  this  a  collimator  and  slit  are  neces- 
sary. But  it  is  not  necessary  to  use  a  narrow  slit  in  taking 
the  eclipse  spectra.  In  1927,  Minnaert  used  a  wide  slit,  and 
there  appears  to  be  no  reason  why  the  slit  should  not  be 
widened  still  further,  perhaps  even  to  take  in  the  whole  of 
the  sun."  In  1932,  it  might  be  added,  Minnaert  employed 
a  modest  little  instrument  without  coelostat  mirror  or  clock 
mechanism.  At  future  eclipses,  the  aluminizing  of  mirrors 
should  permit  spectra  to  be  photographed  to  shorter  wave- 
lengths in  the  violet. 


INDEX 


Abbe,  150,  152,  370 

Abbot,  105,  in,  116,  120,  123,  166, 
174,  182,  391,  396,  406,  411,  419 

aberration,  427 

Abetti,  279,  328 

Abney,  373,  396 

absorption  in  sun's  atmosphere,  116,  147 

absorption  of  water  vapor,  88 

absorption  spectra,  88 

accelerations  of  moon's  motion,  69 

accuracy  needed  in  gratings,  88 

accuracy  of  eclipse  predictions,  67 

Adams,  C.  E.,  224,  376 

Adams,  E.  D.,  199,  202 

Adams,  W.  S.,  73,  283,  289,  292,  29^, 
298,  321,  326,  328,  337,  34S,  374, 
458,  465,  503 

advantages  of  gratings,  104 

age  of  earth,  xiii 

age  of  pyramids,  23 

age  of  sun,  xiii 

airplane  photography,  205,  491 

Airy,  31,  103,  133,  134 

Albrecht,  374 

Aldrich,  397 

alkali  metals,  309 

Almagest,  10,  18,  68 

Ames,  165,  193 

Anderson,  J.  A.,  213,  216,  280,  296 

Anderson,  W.,  415 

Andrews,  394 

Angstrom,  94,  TOO,  101 

anomalous  dispersion,  458 

appearance  of  sun's  surface,  115 

Aristotle,  38 

Arnold,  216,  221 

Arrhenius,  409,  410 

astigmatism,  105 

Aston,  236 

astrology,  17,  18 

astronomical  instruments,  early,  22,  25 

astronomical  unit,  108 

astronomy,  early,  17;  Chaldean,  17; 
Chinese,  17;  Egyptian,  17,  19; 
Greek,  17,  37;  Roman,  18;  Arabian, 
18 

atmosphere  of  moon,  6$ 

atmosphere  of  sun,  116,  147,  149 

atom,  Bohr-Sommerfeld,  239,  242,  276, 
292,  299;  structure  of,  226;  trans- 
formation of,  230;  Rutherford 


model,  234;  positive-ray  analysis; 
236;  mass  confined  to  nucleus,  236; 
chemical  and  physical  properties, 
237;  Lewis-Langmuir,  237;  ex- 
planation of  4686  A,  245 

atomic  numbers,  235 

atomic  weights,  235 

aurora  borealis,  123,  336,  409 

authentic  dates,  19,  20 

Auwers,  123 

Baade,  219 

Babcock,  463,  503 

Babylonian  calendar,  18 

Babylonian  eclipses,  9 

Babylonian  lens,  17 

Bacon,  396 

Baily's  beads,  129,  267 

Baker,  372 

Balmer  series,   240,  243,  244,  245,  309 

Barnard,  159,  165,  174,  182 

Bartoli,  409 

Bassett,  480 

Baume  Pluvinel,  487 

Becker,  391 

Becquerel,  227,  228 

beginnings  of  astronomy,  17 

Belanovsky,  394 

Bergmann,  240 

Bergstrand,  391,  393,  395,  401,  402,  406, 

497,  499 
Bernheimer,  403 
Bessel,  409 
Bigelow,  408 
black  body,  97 
black  drop,  no,  130 
Block,  374 
Bohr,  312 
Bohr-Sommerfeld  atom,  239,  242,  276, 

292,  299 
Bosler,  374 
Bourget,  385 
Bowen,  381,  416 
Brackett,  244 
Bradley,  427 
Bredichin,  409 
Briggs,  396 

bright  line  spectrum,  76,  95,  97 
Brown,  40,  68,  70,  71,  in 
Buisson,  385,  457,  459 


514 


INDEX 


Burns,  280,  348,  352,  374 
Burton,  175,  181 
Burwell,  289 
Butler,  60,  130,  200 

calcium  spectrum,  144 
calculation  of  an  eclipse,  62 
calendar,   of   Chinese,  5;    of   Babylon, 
18;  of  Egypt,  21,  24;  revision  of,  21 
Campbell,  163,  168,  189,  198,  209,  221, 

251,  260,  262,  273,  279,  368,  370, 

374, 375,  376,  377, 4i7,  4^9,  4^4 
Carrasco,  374 
Carrington,  320 
Carroll,  503 
Cavendish,  465 
celestial  latitude,  48 
celestial  longitude,  48 
changes  in  prominences,  261 
changes  in  sun-spots,  84,  109 
Chant,  467 

chemical  elements,  234,  306 
Chester,  183 
Chevalier,  117 
Chinese  eclipses,  i 
Chinese  science,  5 
chromosphere,  108,  135,  147 
chronology,  2,  5,  9,  19,  20,  27 
chronology,  change  of  Biblical  necessary, 

19,  27 

circulation  of  solar  gases,  342,  4°4 
Clerke,  16,  82,  119,  129,  370 
clock,  Egyptian,  22 
Coblentz,  213,  215,  396 
Cohn,  493,  497 
collimator,  254 

color  of  sky  during  totality,  201 
comet  discovered  during  eclipse,  153 
comets'  tails,  409 

composition  of  chromosphere,  353 
composition  of  sun,  72,  98,  100 
composition  of  sun's  atmosphere,  345 
concave  gratings,  104,  248 
Confucius,  6 

constellations,  astronomical,  15,  23 
constitution  of  sun,  72,  98,  100 
contacts,  65,  259 
continuous  spectrum,  97 
Copernicus,  423 
corona,  xv,  23,  108,  131,  139,  140,  143, 

147,  163,  360 

corona  as  Egyptian  symbol,  24 
corona  compared  with  intensity  of  full 

moon,  395 

corona,  intensity  laws,  390 
corona  photographed,  8,  9,  12,  13,  148, 

164,  197,  209,  212,  213,  273,  349, 

364,  365,  400,  425,  448,  449 


corona,  polarization  of,  387,  497 
corona,  shape  of,  364,  399,  417,  497 
corona,  spectrum,   140,   148,    152,   154, 

156,  161,  373,  382,  502,  506 
corona,  total  light,  395,  505 
corona  without  an  eclipse,  155,  418,  501 
coronal  structure  seen  in  different  wave- 
lengths, 371,  373,  380 
coronal    theories,   meteoric,   407,    509; 
mechanical,    407;    electrical,    408; 
magnetic,     408 ;     electro-magnetic, 
408;  radiation-pressure,  409;   elec- 
tron, 414,  508 
coronium,  140,  149,  152,  374,  376,  381, 

4i7,  504 

coronium  lines  in  Nova,  503 
Cortie,  289 
Cottingham,  447 
Courvoisier,  471 
Cowell,  9,  30,  31,  38 
creation,  14 
crescent  lines,  221 
Crew,  165 
Crommelin,  446 
Crookes,  231 
Curie,  xiv,  227 

Curtis,  210,  213,  215,  280,  368,  374,  385 
curvature  of  space,  466 
cylindrical  lens,  80 

darkening  at  limb  of  sun,  116 

dark  lines  in  solar  spectrum,  75,  79,  91, 

95,  97,  141 
date  of  Easter,  37 
dates,  earliest  authentic,  18,  20 
Davidson,  249,  250,  278,  280,  374,  375, 

376,  378,  380,  446,  486 
Debierne,  230 
de  Bruin,  382 

decimal  system  of  Egypt,  21 
delicacy  of  spectroscopic  test,  82 
De  Lury,  321,  322 
density  gradients  in  chromosphere,  284, 

356 

density  of  sun,  112,  123 
de  Sitter,  478,  479 
Deslandres,  105,  139,  161,  373,  418 
development  of  photographs,  363,  495 
development  of  spot,  84,  109 
diameter  of  sun,  48,  in,  123 
diameters  of  eclipse  tracks,  44 
diffraction  of  light,  84,  157 
diffraction  spectrum,  84,  104 
Dingle,  382 
Dinwiddie,  178 

dissociation  theory,  146,  156,  162 
distance  of  moon,  39 
distance  of  sun,  108 
distances  of  stars,  73,  77 


INDEX 


515 


disturbance  in  corona,  209 

D-lines,  82,  93,  96,  137,  307 

Doppler  effect,  340,  343,  384,  456,  458, 

460 

Dorsey,  388 
Dufay,  488,  509 
Duner,  320,  321 
duodecimal  system,  16 
duration  of  partial  phases  of  eclipses,  46 
duration  of  totality,  45 
Dyson,  165,  218,  374,  376,  380 

early  Chinese  records,  5 

earth  a  sphere,  38 

Ebert,  408 

eclipse  of  1652,  360;  1706,  131;  1715, 
128;  1778,  128,  132;  1780,  129; 
1836,  91,  129;  1842,  130,  135;  1851, 
134,  135;  1860,  134,  387;  1868,  137, 
142;  1869,  139,  142,  148,  370;  1870, 
141,  148,  151,  247;  1871,  142,  147, 
148,  151;  1872,  142;  1874,  142; 
1878,  151,  388,  451 ;  1882,  153,  373 ; 
1883,  155,  373;  1886,  158,  373,  396; 
1887,  159;  1889,  159,  396;  1893, 

160,  364,  370,  373,  396,  402;  1896, 

161,  370,  376,  402;  1898,  162,  260, 
370,  373,  376,  396,  402;   1900,  164, 
167,  260,   364,  373,  374,   376,  387, 
402;   1901,  168,  370,  373,  374,  376, 
402;  1905,  183,  251,  252,  260,  374, 
376,  396,  402;   1908,  251,  260,  373, 
374,  376,  396,  402;  1912   (partial), 
125,  287;  1914,  125,  364,  374,  402; 
1916,  364;  1918,  191,  364,  370,  374, 
376,  396,  402;  1919,  421,  444,  446, 
454,461;  1921  (partial),  288;  1922, 
364,   373,  374,  376,  383,   396,  402, 
449,  467;  1923,  126,  203,  370,  402; 
1925,   69,   126,  211,  252,  364,  374, 
376,  396,  402;  1926,  215,  249,  250, 
364,  374,  376,  396,  402;  1927,  216, 
250,  364,  402;  1929,  220,  364,  374, 
376,  402,  474;  1930,  127,  222,  251, 
364,  37i,  374,  376,  402;   1932,  483 
et  seq.;  1934,  492  et  seq. 

eclipses,  Chinese,  i ;  Babylonian,  9 ; 
Egyptian,  23;  Biblical,  27;  classical, 
29;  Thales,  30;  Xerxes,  32;  Julius 
Caesar,  35;  eclipse  of  B.C.,  2137,  i; 
1063,  9;  791,  27;  776,  8;  771,  27; 
770,  27;  763,  10,  19,  27,  54;  720, 
8;  669,  19;  661,  19;  603,  32;  592, 
7;  585,  3i;  552,  7J  549,  7;  495,  7; 
431,  32;  424,  33;  413,  34J  364, 
34;  310,  34 

eclipses  of  past  and  future,  55 


eclipses  that   did  not  occur,  B.C.,  645, 

592,  552,  549,  7J  of  Xerxes,  32;  of 

Julius  Caesar,  35 
eclipses  visible  in  a  year,  58 
eclipses  visible  in  the  United  States,  56 ; 

in  the  British  Isles,  58;  in  Spain,  58 
ecliptic,  inclination  of  moon's  orbit  to, 

39,  48 

ecliptic  limits,  49 
Eddington,  346,  411,  416,  420,  435,  441, 

447,  466,  479 
Egyptian  astronomy,  19 
Egyptian  chronology,  20 
Eichelberger,  174 
Einstein,  205,  421,  443,  479 
electrons,  102,  108,  232 
elements,  234,  306 

elevations,  differences  caused  by,  268 
enhanced,  163,  246,  311 
Epynom  canon,  18 
equivalence,  440 
ether,  425 
Evans,  245 
events,  433 
Evershed,  162,  165,  297,  321,  326,  340, 

343,  349,  458,  460,  464,  467 
evolution,  72,  113,  318 
excitation  potential,  313,  339 
Exner  and  Haschek,  105,  303 
expenses  of  expeditions,  139 

Fabry,  385,  396,  412,  457,  459 

faculae,  118,  123 

Faye,  320,  322 

first  photograph,  135 

Fisher,  492 

Fitzgerald,  427 

flash  spectrum  at  partial  eclipses,  287 

flash  spectrum,  discovered,  141 ;  photog- 
raphy necessary,  147 ;  improvements 
in  observing,  157;  photographed  in 
1896,  161;  photographed  in  1898, 
162;  enhanced  lines,  163;  gratings 
used  in  1900,  168;  advantages  of 
various  spectrographs,  248;  method 
of  timing  photographs,  258;  mov- 
ing plate  vs.  fixed  plate,  261 ;  jump- 
ing film,  489;  intensities  compared 
with  Fraunhofer  spectrum,  268, 
309;  determination  of  heights,  271; 
effects  of  seeing,  274;  discussions 
of,  277  ;  self -absorption  in,  283,  286; 
density  gradients,  284,  356;  at  a 
partial  eclipse,  287;  general  con- 
clusions, 2Q3 ;  recommendations  for 
future  eclipses,  294,  510;  without 
an  eclipse,  289 

flocculi,  108,  300 


516 


INDEX 


focus  of  eclipse  spectra,  248 

Forbes,  91,  130 

Fotheringham,  3,  9,  30,  31 

Foucault,  93,  100 

four-dimensional  space,  422,  466 

Fowle,  123 

Fowler,  105,  160,  241,  245*  275,  276,  287, 
314,  343,  373,  374,  376,  378 

Fox,  92,  134,  209 

Fraunhofer,  76,  101,  123 

Fraunhofer  lines,  75,  79,  9*»  95,  136,  143, 
152,  268,  269,  275,  277,  281,  293, 
303,  350,  382,  383,  406,  407,  506 

Freeman,  381 

Frerichs,  382,  504 

Fresnel,  425 

Freundlich,  216,  220,  475,  477 

Frost,  165,  374 

furnace  lines,  315 

future  eclipses,  55,  510 

Gale,  458 

Galileo,  423 

Gallo,  210 

Gare,  396 

Geiger,  231,  234 

geodesic,  435 

Gill,  no 

Graff,  391,  396,  401 

granulation,  116 

gratings,  84,   104,   157,  168 

gratings  and  prisms  compared,  104,  248 

grating  spectrum,  85,  104 

gravitation,  421,  443 

Grotrian,  312,  374,  375,  376,  378,  379, 

5o6,  509 
Grouiller,  488,  509 

Hale,  105,  139,  289,  328,  331,  4i8 

Hall,  141,  455 

Halley,  128,  131,  134,  451 

Halm,  321,  322,  323,  325,  458 

Hammond,  197 

Hansen,  31,  68,  71 

Hansky,  117,  367,  418 

Harkness,  140,  141,  151 

Hartmann,  79 

Harwood,  393 

Hastings,  156,  481 

Hayn,  125,  126 

heat  of  corona,  174 

heights  of  solar  vapors,  165,  271,  319 

heliometer,  77,  no,  123 

helium,  xiv,  137,  239,  270,  309,  359 

Herodotus,  reliability  as  historian,  32 

high-level  lines  in  sun,  304 

high-temperature  lines,  308 

Hills,  161,  162,  374,  376 

Hind,  31,  63 


Hipparchus,  17,  31,  38 

historian,  2,  32 

Holden,  396 

Hooke,  425 

Hopfield,  382 

Horn  d'Arturo,  216,  369 

Hubble,  414 

Hubrecht,  321 

Huff,  168 

Huggins,  103,   138,   155,  158,  239,  408, 

418 

Hull,  409 

Humphreys,  105,  165,  457,  481 
Hussey,  368 
hydrogen,  136,  149,  152,  239,  270,  310, 

355 
Hylleraas,  382 

intensities  in   flash   spectrum,   268,   304 
intensities  of  spectra  of  electric  arc  and 

sunlight,  93 

intensity  of  coronal  light,  395,  506 
intra-Mercurial  planets,  153,  174 
invisible  sun-spots,  332 
ionization,  299,  311 
ionized  atoms,  233,  246,  311 
irradiation,  124,  126,  127 
isotopes,  235 

Jackson,  475,  486 

Janssen,    68,    103,    116,    137,    141,    i52> 

156 

Jeffreys,  455 

Jewell,  105,  174,  178,  254,  456,  457 
Johnson,  221,  493,  510 
Jones,  467 
Joy,  503 
Julius,  458 

Kaufmann,  228 

Kayser,  105,  240,  253,  275,  282,  425 

Kelvin,  421,  425 

Kepler,  18,  73,  131,  423,  451 

Kimura,  120 

King,  A.  S.,  105,  314,  345 

King,  E.  S.,  213,  393,  397 

King,  L.  W.,  9,  12 

Kirchhoff,  73,  92,  94,  99,  *35,  *44 

Kirchhoff  laws,  97 

Knopf,  396 

knowledge,  how  derived,  434 

Kohlschiitter,  210 

Krakatoa,  170,  411 

Kunz,  395,  396,  419 

Lalande,  451 

Langley,  105,  115,  118,  141,  iS1*  i°8 

Larmor,  120,  427,  466 

law  of  coronal  intensity,  390 


INDEX 


517 


law  of  diffraction  spectra,  85 

law  of  gravitation,  421,  443 

laws  of  spectrum  analysis,  97 

Lebedew,  409 

length  of  month,  40 

Lemaitre,  479 

length  of  seasons,  39 

length  of  shadow  cast  by  earth,  42 

length  of  shadow  cast  by  moon,  43 

length  of  year,  40 

Lescarbault,  451 

Leuschner,  396 

level  of  sun-spots,  119 

levels  at  which  spectral  lines  originate, 

3i9,  342,  350 
levels  in  chromosphere,  165,  277,  319, 

33? 

Leverrier,  451 
Lewis,  374,  376,  380,  390 
Lewis-Langmuir,  237 
Lick  Observatory,  18,  33,  160,  163,  167, 

189,   198,   209,   221,  260,  261,  264, 

265,   266,  278,  361,  367,  372,  383, 

384,  410,  467,  489,  503 
life  on  earth,  xiii 
light-pressure,  409,  445 
light-waves,  101 
Lindemann,  418 
Lockyer,  90,  103,  135,  139,  142,  143,  144, 

152,   161,   162,  247,  261,  317,  374, 

376,  378 

Lockyer,  W.  J.  S.,  163,  165,  402 
Lohse,  217 

long  and  short  lines,  145 
Lorentz,  427,  437 
low-pressure  lines,  308 
Ludendorff,  210,  401,  402,  415,  497,  499, 

506 

luminosity  of  corona,  157,  395 
lunar  ecliptic  limits,  49 
lunar  tables,  68 
Lyle,  105 
Lyman,  244 
Lyot,  419,  501,  504 

magnetic  field  of  sun,  330,  335 

magnetic  storms,  123,  336 

Marriott,  127,  224,  225,  362,  371,  380, 

401,  495 
Marsden,  234 
mass  of  sun,  112 
Maunder,  23,  163 
Maxwell,  409,  425,  445 
Mees,  193 

Meggers,  105,  276,  314 
Melotte,  467 
Menzel,  260,  261,  267,  275,  279,  280,  284, 

347,  353,  355,  35$,  49° 


Mercury,  424,  437,  443,  451 

Merfield,  105 

meteoric  hypothesis,  150 

meteors,  114,  149 

meter,  109,  432 

method  of  viewing  sun,  115 

Metonic  cycle,  37 

metric  system,  108 

Michelson,  105,  120,  207,  425 

Miller,  D.  C.,  426 

Miller,  J.  A.,  10,  210,  212,  220,  361,  369, 
370,  410,  484,  485,  495 

Miller,  W.  H.,  92,  103 

Millikan,  243 

Milne,  300,  353,  35»,  4*4,  4*6,  4^3 

Minnaert,  219,  225,  250,  251,  273,  278, 
281,  282,  283,  353,  356,  484,  5°5> 
Sio,  511 

Minkowski,  434,  437 

Mitchell,  W.  M.,  329 

Mohler,  457 

month,  Babylonian,  18;  Egyptian,  21; 
length  of  different,  40 

moon  away  from  predicted  place,  67 

moon,  distance  of,  39 

moon  interferes  with  astronomical  ob- 
servations, 44 

moon,  motion  of,  39,  68 

moon's  shadow,  284,  485,  491 

Moore,  C.  E.,  283,  314,  345 

Moore,  J.  H.,  370,  372,  374,  375,  37&» 
377,401,506 

Morley,  426 

Moseley,  234 

motion  in  chromosphere,  342 

motion  in  corona,  365,  3^4 

motion  in  line  of  sight,  73,  342,  464 

motion  of  the  moon,  39,  68 

motions  of  solar  gases,  342,  464 

moving  plate  spectrograph,  260 

moving  plate  vs.  fixed  plate,  261,  271 

multiplet  groups,  314,  344,  348 

music  of  the  spheres,  37 

Naegamwala,  374 

nebulae,  413 

nebulium,  416 

neutral  atom,  307 

Newall,  162,  166,  182,  217,  288,  322,  325, 

374,  376,  390 
Newcomb,  31,  36,  41,  54,  62,  68,  123, 

141,  152,  405,  424,  452 
Newton,  73,  101,  407,  421,  423,  446 
Nichols,  409 

Nicholson,  J.  W.,  381,  393 
Nicholson,  S.  B.,  in,  120,  213,  337,  39$ 
Nineveh,  eclipse,  8,  n,  18,  26,  54,  70 
nodes,  39 


518 


INDEX 


normal  spectrum,  85,  104 
northern  lights,  123,  336,  409 
number  of  eclipses  each  year,  59 
number  of  eclipses  in  Saros,  52 

objective  prisms,  82 

Oppolzer,  i,  29,  36,  51,  60,  62,  63 

Pannekoek,  219,  250,  251,  273,  278,  281, 

282,  283,  353,  356,  381 
parallax  of  the  moon,  48 
parallax,  solar,  48,  no 
parallax,  stellar,  73,  77,  124 
Parkhurst,  213,  396 
partial  eclipse,  46 
Paschen,  244,  245 
past  eclipses,  55,  494 
Payne,  347 

percentage  of  ionization,  301 
Perepelkin,  394 

periodic  table  of  elements,  234,  306 
period  of  sun-spots,  119 
Perrine,  173,  181,  367,  368,  370,  383,  338, 

39^ 

personal  errors,  124 
Pettit,  213,  393,  396,  502 
photographing  the  sun,  115,  125 
photography  first  applied,  134 
photosphere,  98,  113 
Pickering,  140,  141,  244,  396 
Pitman,  127 
Planck,  238,  242 
plane  gratings,  104 
Plaskett,  H.  H.,  321,  353 
Plaskett,  J.  S.,  209,  321 
polarity  of  spots,  331,  333 
polarized  light,  148,  387,  509 
pole-effect,  459 
Poor,  453 

potassium  in  sun,  308 
prediction   of   eclipses,    5;    Thales,   31; 

from    ecliptic    limits,    50;    by    the 

Saros,  31,  50;  from  Oppolzer  maps, 

Si 

pressure  in  photosphere,  114 
pressures  in  sun's  atmosphere,  307,  346 
principles  of  spectrum  analysis,  97 
prism,  73 

prismatic  camera,  82,  153 
prismatic  spectrum,  85,  104 
prisms  compared  with  gratings,  104,  248 
problems  of  the  corona,  360,  506 
Proctor,  23 
prominences,  25,  123,  128,  136,  149,  158, 

165,   196,  261,  272,  320,  321,  369, 

370,  401 
protons,  232 
Ptolemy,  10,  19 
pyramid  not  an  observatory,  23 


quantum,  102,  238 

Radau,  68 

radiation  of  corona,  166,  395 

radiation-pressure,  409,  445 

radium,  xiv,  228,  316 

Ramsay,  230 

Ranyard,  399 

Rawlinson,  n,  18 

red  flames,  131,  136 

relative    numbers    of    lunar    and    solar 

eclipses,  61 
relative   numbers   of   total  and  annual 

eclipses,  44 
relativity,  205,  215,  221,  280,  349,  421, 

443 

reversal  of  polarities,  333 
reversed  spectrum,  97,  98 
reversing  layer,  98,  141,  143,  147,  163, 

247 

Ricco,  418 
Rigge,  64 
Rontgen,  227 
Rosenthal,  382 
Ross,  397,  471 
Rosseland,  217,  358,  508 
rotation  of  corona,  161,  164,  182,  384, 

504 

rotation  of  sun,  319 
rotation  of  sun's  magnetic  axis,  336 
Rowland,  H.  A.,  88,  103,  113,  162,  168, 

205,  282,  456 
Rowland,  J.  P.,  289 
Rowland  gratings,  104,  390 
Royds,  230,  321,  460 
rubidium  in  sun,  308 
Runge,  105,  240,  253 
Russell,  105,  126,  276,  283,  300,  308,  314, 

345,  349,  354,  381,  449 
Russell,  Dugan  and  Stewart,  241 
Rutherford,  228,  230,  231,  242 
Rydberg,  105,  240,  241 
Rydberg  constant,  240,  241,  243 

Sana,  268,  270,  276,  292,  294,  299,  315, 

4i3 
St.  John,  105,  iii,  321,  322,  326,  340, 

341,  344,  348,  352,  374,  463 
Saros,  17,  31,  37,  41,  50,  53,  62 
Satome,  493,  497 
Saunders,  105,  252,  314 
scale  of  intensities,  354 
scattering  of  light,  387,  412 
Schaeberle,  159,  370,  407,  410 
Schlesinger,  321 
Schorr,  210,  218 
Schuster,  120,  154,  373 
Schwabe,  119,  133 
Schwarzschild,  302,  391,  396,  415,  416 


INDEX 


519 


seasons,  length  of,  39 

Secchi,  103,  135,  387 

seeing,  274 

sensitiveness    of   spectroscopic   test,   83, 

232 

series,  in  spectra,  240 
Shackleton,  161,  376  / 

shadow  bands,  480  J 

shadow  cast  by  earth,  length  of,  42  ^  A 
shadow  cast  by  moon,  length  of,  43       <  f 
shadow  of  moon  approaching,  284 
shadow,  width  of,  44 
shape  of  corona,  123,  364,  399,  417,  497 
shape  of  sun,  124 
shift  to  red  in  spectrum,  457 
sierra,  134 
size  of  atom,  237 
size  of  electron,  226 
size  of  molecule,  233 
size  of  universe,  226 
Skinner,  174 
Slipher,  374 

slitless  spectroscope,   82,   149,   153,   250 
Slocum,  76,  80,  138,  139,  449,  502 
Soddy,  230 

sodium  lines  conspicuous,  82 
solar  ecliptic  limits,  49 
solar  parallax,  48,  no 
source  of  knowledge,  434 
space  curved,  466 
space  of  four  dimensions,  422,  466 
spectral  series,  240 
spectroscope,  72 
spectroscopic  binaries,  73 
spectroscopic  tests,  sensitiveness  of,  83, 

232 

spectrum  analysis,  principles,  97 
spectrum  lines  on  dark  moon,  154 
spectrum  of  corona,  140,  148,  152,  154, 

156,  161,373,382,504 
spectrum  of  Venus,  81 
spherical  sun,  124 
spot-vortices,  329,  340 
Stark-effect,  285,  336,  459 
Stebbins,   209,   213,  368,  395,  396,  419 
stellar  parallax,  73,  77,  123 
stellar  spectrum,  80,  317 
Stetson,   213,   215,   217,   221,   394,  396, 

397 

Stevens,  491 
Stewart,  94,  100 
Stokes,  93,  100 
Storey  and  Wilson,  321 
Stormer,  217 
Stratton,  225,  249,   250,   278,  280,  374, 

375,  376,  378,  380,  484 
stripped  atoms,  246 

sun-spot  photographs,  109,  117,  320,  333, 
340 


sun-spots,  112;  appearance,  117;  level 
of,  119;  period  of,  119;  distribution 
of,  121,  334;  related  to  other  phe- 
nomena, 121 ;  spectrum,  308;  vor- 
tices, 329;  polarity,  331,  333; 
invisible  spots,  332 

surface  of  sun,  115 

Swift,  153 

Tacchini,  139,  158 

telluric  lines,  90 

temperature,  change  of  spectrum  with, 

144,  145 

temperature  change  during  eclipse,  492 
temperature  of  sun,  114,  302,  308,  346 
temperature  of  sun-spots,  308 
temperatures  of  stars,  72,  145 
tenth-meter,  101 
terrestrial  magnetism,  122 
Thales,  17,  30,  70 
theory  of  eclipses,  42 
Thomson,  232,  236 
Thorpe,  396 

time  available  for  observations,  xvii 
times  of  contacts,  66,  259 
titanium  oxide,  bands  of,  343 
Todd,  161 

transit  of  Venus,  no,  130 
Trumpler,  468,  475 
Turner,  158,  162,  166,  391,  396 
Tut-ankh-Amen,  24 
Tycho  Brahe,  128,  423 

Ueta,  493 

ultimate  lines,  315 

unexplained  facts  about  corona,  406 

Unsold,  286,  3S3 

valence,  237 

variable  star,  120 

Vegard,  219 

velocity  of  light,  in 

velocity  of  moon's  shadow  over  earth, 

45 

von  Brunn,  476,  477 
von  Kluber,  369,  394,  401,  402,  475,  477 
von  Soldner,  465 
vortices,  sun-spot,  329,  340 

Ware,  321,  326,  341,  374 

Watson,  141,  153 

wave-lengths,  79,  87,  97,  101,  105,  461 

weather  and  sun-spots,  121 

week  of  seven  days,  18;   of  ten  days, 

21 

Wesley,  166,  365 
Weston,  505 
wet-plate  photography,  115 


520  INDEX 

Weyl,  442  year,    Babylonian,    18;    Egyptian,    21; 

white  prominences,  158  tropical,  21;  Julian,  21;  Gregorian, 

Williams,  E.  T.  R.,  355,  356,  401  21;  length  of  various,  41 

Williams,  J.,  9,  134  Young,  C.  A.,  97,  103,  113,  139,  *47,  H9> 

Wilson,  A.,  119  152,  158,  168,  247,  260,  329 

Wilson,  C.  T.  R.,  233  Young,  R.  K.,  388,  391,  467 

Wilson,  H.  C.,  209,  279 

Wollaston,  76  Zanstra,  416 

Wood,  244,  387,  389,  418  Zech,  31 

Wooley,  353  Zeeman,  105,  329,  459 

Wright,  A.  W.,  388  zodiac,  15,  26 

Wright,  W.  R.,  3»S,  477,  478,  495  zodiacal  light,  409,  454 


COLUMBIA  UNIVERSITY  PRESS 

COLUMBIA  UNIVERSITY 
NEW  YORK 


FOREIGN    AGENT 

OXFORD  UNIVERSITY  PRESS 

HUMPHREY  MILFORD 
AMEN  HOUSE,  LONDON,  E.  C.  4